Method of effecting fast turbine valving for improvement of power system stability

ABSTRACT

As a improved way of effecting fast valving of turbines of power system steam-electric generating units for the purpose of improving the stability of power transmission over transmission circuits to which their generators make connection, when stability is threatened by line faults and certain other stability endangering events, the heretofore employed and/or advocated practice of automatically closing intercept valves at fastest available closing speed in response to a fast valving signal, and thereafter automatically fully reopening them in a matter of seconds, is modified by providing to reopen the valves only partially to and thereafter retain them at a preset partially open position. 
     For best results the process of what can be termed sustained partial reopening is so effected as to result in its completion within a fraction of a second following the peak of the first forward swing of the generator rotor. 
     Control valves may be either held open, or automatically fully or partly closed and thereafter fully opened in a preprogrammed manner, or automatically moved to and thereafter held in a partly closed position, by means of a preprogrammed process of repositioning in which the valves may optionally be first fully or partly closed and thereafter partly reopened. 
     Avoidance of discharge of steam through high pressure safety valves can be had with use of suitably controlled power operated valves that discharge steam to the condenser or to atmosphere. 
     Where there is an intermediate pressure turbine that is supplied with superheated steam, use of sustained partial control valve closure, if employed, is supplemented by provision .Iadd.for reduction of .Iaddend.rate of heat release within the steam generator in order to protect the reheater from overheating. .Iadd. 
     As a way to restrict increase of reheat pressure of fossil fuel installations, and to minimize increase in the MSR (moisture separator-reheater) pressure of nuclear units, provision is optionally made of normally closed by-pass valves which are arranged to accept reheat or MSR pressure steam and discharge to the condenser in response to either an an increase of reheat or MSR pressure that takes place other than slowly, or to a fast valving signal, or both. .Iaddend.

CROSS REFERENCE TO RELATED INVENTIONS

My invention relates in its principal aspect to means for rapidlycontrolling power flow within power transmission elements ofinterconnected power systems with a view of favorably affecting thestability of such systems when jeopardized by suddenly occurring adverseevents. This patent application is subject matter related to my issuedU.S. Pat. Nos. 3,051,842, R26,571, 3,515,893, which has reissued as U.S.Pat. No. R27,842, and U.S. Pat. No. 3,657,552, and is a continuation inpart of my copending applications, Ser. No. 244,594 filed Apr. 17, 1972,and Ser. No. 388,619 filed Aug. 15, 1973, which have since issued asU.S. Pat. Nos. 3,849,666 and 3,848,138 respectively.

BACKGROUND OF THE INVENTION

1. Field of Invention

The area of utility of the invention comprises prevention of developmentof system instability within power systems when threatened bytransmission line faults, and certain other system stability endangeringevents.

The area of method comprises responding to faults, and other events thatcould endanger system stability, by rapidly initiating preprogrammedprocesses of

a. full or partial fast closure of intercept valves of steam turbinetype generator prime movers of power systems, preferably effected within1/4 second,

b. subsequent partial reopening of intercept valves, preferably soeffected that the valves begin to reopen somewhat in advance ofoccurrence of the peak and the first forward swing of the generatorrotor, and substantially attain planned full extent of partial reopeningwithin a fraction of a second following that peak.

The preprogrammed processes (a) and (b) may optionally be supplementedby other control measures, such as, but not limited to, control valverepositioning and initiation of change in rate of steam generation bysteam supply sources, but employment of such supplemental measures isnot requisite.

2. Prior Art

This invention is similar to, but can be viewed as, in certain aspects,more basic than that disclosed in the writer's copending applicationSer. No. 388,619, now U.S. Pat. No. 3,848,138.

To ensure adequate description of the prior art, the presentationcontained in U.S. Pat. No. 3,848,138 is to be regarded as incorporatedin this application by reference.

To aid in clarifying how the prior art relates most closely to thisapplication, it has appeared to be desirable to review what was involvedin the total process of invention under several headings, as in whatfollows.

A. INVENTION STATUS OF 1962

In the writer's U.S. Pat. No. 3,051,842 he disclosed the concept ofpreprogrammed control valve closure followed by partial reopening, andin his U.S. Pat. No. R26,571 added the concept of also rapidly closingand thereafter fully opening intercept valves, but he did not discusssteam generator controls, or the behavior of safety valves.

B. STEAM GENERATOR CONSIDERATIONS

Actually, where what was being dealt with was fossil fuel fired steamgenerators, of U.S. design, supplying superheated steam to both high andintermediate pressure steam turbines, there was inherent a need toreduce fires, as in a matter of a minute, following a first sustainedreduction of high pressure turbine steam acceptance, effected as aresult of fast valving, in order to protect the reheater fromoverheating.

Also during this 1 minute period the high pressure safety valves wouldbe discharging steam to atmosphere, except to the extent that occasionfor them to do so would be avoided, or minimized, as a result of theoperation of often, but not always, provided, power operated, or socalled "powermatic," valves that are arranged to open in response to arise in steam pressure, and that provide a substitute means of diversionof steam to atmosphere.

Also where, as the writer discovered, typically, provision was not beingmade for opening intercept valves faster than in a matter of 10 seconds,it could apply that if control valves were not closed sufficiently,reheat pressure safety valves could also lift and discharge steam.

Further, the point came up that when steam discharges through safetyvalves, particles of metal that would be carried over from the boiler,or the superheater, or the reheater, could cause damage to the sealsurfaces, and prevent perfect reseal when the valves reclosed, with theeffect that it would be necessary to schedule a unit shutdown to allowrepairs, which sort of thing typically would involve considerableexpense, in view of the unit being taken out of service, and therequirement of temporarily generating substitute power with older andless efficient machines.

Now, actually, as see reference 69 of the table of references, itappears to be possible to so thoroughly clean boilers, superheaters, andreheaters, that safety valves are seldom if ever damaged, when theydischarge steam, which approach is, or has been, successfully used byOntario-Hydro, as a way to prevent safety valve damage on trip-off toauxiliary load, which they have commonly employed when tripping a unitoff the line.

Also, it can be argued, that, at least when fast turbine valving is tobe seldom invoked, it would be possible to merely tolerate the damagethat lifting of safety valves induces, which, it may be noted, is thepolicy that applies at the Four Corners station of Arizona PublicService, where a 750 mw unit is typically tripped off under load fourtimes a year, in response to line faults, as a way to avoid loss ofintersystem ties on the occurrence of permanent type faults (cf ref.25).

However, considering the attitude of those power system engineers whoare directly concerned with the operation of power plants, it becameincumbent on the writer to seek solutions to the problem of safety valvelifting, and, with this in mind, when the original of U.S. Pat. No.R27,842 was written, he included in it, as see par. 3 of column 24, thestatement

"I propose to avoid operation of the high pressure safety valves byproviding fast acting and fully commercially available dump valves,which are programmed by the fast turbine control system to dump highpressure steam, either to atmosphere, or preferably, to the turbinecondenser, with concomitant supply of spray water for cooling purposes."the idea being to employ enough dump valves to eliminate lifting ofsafety valves.

When it came to valves for dumping high pressure steam, powermatic,pilot operated type valves were already commercially available, whilealso it is feasible to provide shut off valves ahead of them, which,when done, allows repair without need to schedule a unit shutdown.

However, there was a question as to what to employ when it came toproviding dump valves for use at reheat pressure.

In 1964, in discussion of this problem with personnel of the CrosbyValve Co. of Wrentham, Mass., the point came out that safety valves canbe converted to power operated valves, and hence to "dump valves," byequipping them with air operated lifting devices which, it was claimed,would cause the valves to lift in a fraction of a second, while thepoint also applied, that if used to supplement safety valves, thesevalves could also be mounted downstream of shut-off valves.

Thus when the original of U.S. Pat. No. R27,842 was written the writerknew how to proceed to make available normally non-leaking valves fordumping both high and reheat pressure steam to atmosphere, withprovision to allow maintenance when required, absent need to schedule aunit shutdown.

C. CONCEPT OF SUSTAINED PARTIAL INTERCEPT VALVE REOPENING

As it turned out the availability of non-leaking bypass valves, adaptedfor use at reheat pressure, led the writer to the idea that it would bepossible, and that it could also be desirable, to have recourse torestricting the extent of reopening of intercept valves, whereby toeffect a sustained reduction of turbine driving power by this means, andit was with this in mind that he included in the application for theoriginal of U.S. Pat. No. R27,842 the statement (cf par. 7 column 24)

"Also, it will be clear that where a sudden, sustained drop in drivingpower of a reheat type turbine is wanted, it will be only feasible toachieve desired results by using the intercepting valve as a steam flowmodulation device to supplement control valve modulation of steam flowto the high pressure turbine which, however, it is judged can beaccomplished by those skilled in the art merely with application ofgenerally known practices (21, 22)."

However since the statement does not describe what, in detail, would bedone, it is not thought to anticipate either what is disclosed incopending application 388,619, now U.S. Pat. No. 3,849,666, or theconcept of effecting a type of preprogrammed intercept valve reopeningthat would terminate valve motion at a partly open position, that iscovered in the present application.

The new concept of the present application, it may be noted, offerscertain advantages in the way of simplicity, and can be used where

a. as in Continental European type once-through fossil fuel and alsoHTGR, high temperature gas cooled reactor type nuclear installations,the high pressure turbine is provided with a by-pass to the cold reheatline,

b. in those nuclear installations in which extent of reheat is slight,and there is therefore no problem of protection of the reheater fromoverheating,

c. when control valves either are held open, or are first rapidly fullyor partly closed, and then fully opened,

as also when

d. sustained .Iadd.partial .Iaddend.control valve closure is employedand is supplemented by initiation of a reduction in rate of steamgeneration within the steam supply source,

but the notable feature is that sustained driving power reduction can beeffected without need to program a change in steam generation when anyof control procedures (a), (b) or (c) above are made use of, a featureof importance since, with use of these techniques, when a faulted linehas been opened on a temporarily sustained basis, either as a matter ofgeneral policy, or because of development of a refault on automaticcircuit breaker reclosure, if and when the line is, perhaps, quicklyrestored to service, it becomes possible to reestablish full generatoroutput in a matter of seconds, since there had been no need for recourseto the slow and also slowly reversed process of change in rate of steamgeneration.

D. HISTORY OF THE CONCEPT OF EMPLOYING LOW PRESSURE STEAM BY-PASS VALVESAS A WAY TO SOLVE THE PROBLEM OF FAST VALVING TURBINES THAT RECEIVESTEAM FROM BOILING WATER TYPE REACTORS

Beginning in April 1966 the writer endeavored to interest CommonwealthEdison in equipping two GE nuclear type turbines with provision for fastturbine valving.

This led, in due course, to stability studies that demonstrated abilityto deal with the problem of delay in fault clearance.

However, when it came to fast valving these turbines, which were to beinstalled in a power station named Quad Cities, studies carried out byGE Schenectady, which were based on purely momentary intercept valveclosure, established that a problem would develop, in that, due to slowreopening of intercept valves, it could be predicted that pressure aheadof the low pressure turbine would rise when fast valving was invoked,and this, in turn, would cause a rise in the pressure ahead of the highpressure turbine, that would cause a hazard of scramming the reactor,which was of the BWR, or boiling water type, that GE was producing atSan Jose.

On looking into what was involved, the writer determined that it hadbecome GE San Jose practice to use spring loaded valves, of a modifiedtype, which Crosby Valve had been producing, as a means of discharging1,000 psi steam to the containment vessels of its BWR reactors, themodification consisting in applying a bellows which would seal the pathof discharge steam and, with the valve in closed position, preventleakage of air into the condenser.

It then occurred to the writer that these valves could be converted to afast acting power operated type by equipping them with air operated liftcylinders, and that they could be used as a way to solve the Quad Citiesunits fast valving problem by providing so that they would respond to afast valving signal by popping open and discharging to the condenser,either,

a. high pressure steam, or

b. low pressure steam.

To progress this idea further Crosby Valve agreed to carry out testswhich would demonstrate speed of operation, and this was done, first ona small valve in July of 1967, and later, in November, on a largervalve, of the size used in nuclear installations.

GE had previously considered using air lifted valves in a BWRinstallation but had given up the idea on the assumption that they wouldnot be fast enough.

The November test, which was witnessed by GE personnel, showed that thevalves could be opened in less than 1/10th second, while also CrosbyValve planned to step up speed to 1/20th second, which was judged by GEto be sufficiently fast.

The November test led to GE San Jose thereafter incorporating the airlift feature as an element of its high pressure relief valves.

Also San Jose accepted that use of air lift valves ahead of either thehigh or low pressure turbine could be used as a way to prevent reactorscram as a result of use of fast valving, but, as it turned out,Commonwealth Edison decided against use of fast valving on the theorythat it might cause problems that would prove to be a source ofdifficulty and so nothing was done.

However the idea of utilizing a power operated fast acting low pressurerelief valve as a way to limit rise of pressure ahead of the lowpressure turbine, and hence, also ahead of the high pressure turbine andwithin reactors of BWR type, when fast valving was employed, remained asa presumably entirely workable concept.

Also as early as November 1967 the writer brought to the attention ofthe Crosby Valve Co. the point that a market could develop for valvesthat would by-pass around both low pressure turbines of nuclear unitsand intermediate pressure turbines of fossil fuel steam source type, asa way to allow fast valving of the sustained reduction of driving powerversion.

In continuation, in 1969 the writer took up with Crosby Valve the matterof the cost of equipping the spring loaded low pressure relief valves ofTVA's Browns Ferry BWR reactor type nuclear steam electric units withair lifters.

This was at a point when TVA was giving consideration to use of fastvalving of the sustained reduction of driving power type at BrownsFerry, something that they were deflected from as a result ofrepresentations by GE Schenectady as to the possibility of problems withthe drain system of the moisture separators.

Rather than provide to fast valve at Browns Ferry, TVA decided to askfor this feature as an option in the case of two 1300 mw nuclear unitsthat were to be installed in a station at Watts Bar.

Since Westinghouse was a bidder and could provide sustained reduction ofdriving power with use of its PWR pressurized water reactors, which wereequipped with 45 percent high pressure by-pass systems, and sinceWestinghouse was not prepared to hold its intercept valves in a fixedmodulating position, the concept of providing for use of sustainedpartial intercept valve reopening was not raised, and this also appliedwhen it later came to a similar nuclear station that was to be locatedat Bellefonte, and that would utilize a Babcock & Wilcox PWR typenuclear reactor.

In 1973 the question again came as to the feasibility of fast valving aTVA BWR installation, this time in relation to a plant to be located atHartsville which would incorporate four 1220 mw turbines that would besupplied with steam from GE BWR type reactors.

The Hartsville turbine award went to Brown Boveri, which concern quotedon provision of fast turbine valving of the sustained reduction ofdriving power type as an extra cost option.

Brown Boveri has, since the award, been in touch with GE San Jose, andreportedly there has been consideration of employing air lifting ofspring loaded low pressure relief valves as a way to avoid a reactorscram, but, to my knowledge, there has been no published account ofemployment of sustained partial reopening of intercept valves as a meansof effecting sustained reduction of driving power type fast valving inBWR steam electric installations.

E. HISTORY OF THE CONCEPT OF USE OF SUSTAINED PARTIAL LIFTING OFINTERCEPT VALVES AS A MEANS TO PROVIDE FOR FAST VALVING OF THE SUSTAINEDREDUCTION OF DRIVING POWER VERSION IN THE CASE OF REHEAT TYPE FOSSILFUEL STEAM ELECTRIC INSTALLATIONS

The concept of utilizing electrically controlled air assisted springloaded valves as by-pass valves that would discharge steam from a pointjust ahead of an intermediate pressure turbine of a fossil fuel steamelectric installation to make possible sustained partial lifting ofintercept valves as a way to accomplish sustained driving powerreduction type fast valving, yet avoiding lifting of reheat pressuresafety valves, was gone into with Crosby Valve as early as the latterpart of April 1964 though this was in the context of using these valvesto divert steam to atmosphere for the period of time required for fastas feasible reduction of boiler fires, since, at that time, these valveswere not understood to be capable of being adapted to allow thedischarge of steam to the condenser.

It was recognized that other types of fast acting valves, that could beused to discharge to the condenser, were available, but engineers ofEbasco Services, with which contact was established in 1966, took theposition that leakage would represent an insurmountable obstacle totheir use.

However, the availability, as of 1967, of sealed type spring loadedvalves such as were developed for use in connection with BWR nuclearinstallations, coupled with the approach of installing isolation valvesaround these valves as a way to allow repair without scheduling a unitshutdown, provided a way out of this difficulty.

Standing in favor of sustained partial intercept valve lifting is thefact that it can be used as a way to either avoid or minimize need toreadjust rate of steam generation, while standing against it is theexpense of providing the valves and their shutoffs.

At the 1969 American Power Conference, a Siemens paper (35) describedthe German practice of regularly equipping oncethrough boiler steamelectric installations with by-pass systems that diverted steam botharound the high pressure turbine to the cold reheat line and from thehot reheat line to the condenser, as both control and intercept valveswere simultaneously partly closed in response to change in speed in theevent of sudden partial loss of load which at once made clear that, whenit came to Germany, sustained driving power reduction type fast valvingfor stability improvement purposes, would be very easy to provide.

However, as also brought out in the statement of the prior art containedin the writer's application Ser. No. 388,619, now U.S. Pat. No.3,848,138, the writer took up with Siemens and later with M.A.N., theidea of providing response to line faults as a means of improving systemstability, while the idea was viewed as certainly new, it was notaccepted as providing a solution to an economic need.

Also, although an article on electrohydraulic turbine control systemsthat appeared in a recent issue of Elektra (66)* discusses fast valving,(as see sections 3.4 and 5.8), and stresses its potential importance,though techniques of fast valve closure and fast full reopening arecited, no mention is made of partial reopening of either control orintercept valves, and while there is, in the U.S. literature referenceto sustained partial opening of control valves after fast closureeffected for purposes of improving power system stability (21, 22, 23,25, 67), the writer fails to recall any instance of published referenceto sustained partial opening of intercept valves following an initialprocess of closure in the context of a measure intended for improvementof stability.

F. PROVISION TO PROVIDE AGAINST DEVELOPMENT OF INSTABILITY OF DRAINSYSTEMS OF MOISTURE SEPARATORS, AS A CONSEQUENCE OF APPLICATION OF FASTTURBINE VALVING TO BWR AND PWR TYPE NUCLEAR STEAM ELECTRIC INSTALLATIONS

When it comes to BWR and PWR type nuclear installations GE Schenectadyhas stressed the fact that fast valving might cause objectionableinstability of moisture separator reheater drain systems.

Here the point that applies is that decrease in MSR pressure tends tocause the flashing into steam of water contained in the drain system,and can result in a surge of drain water back into the MSR.

This is a problem that also develops when the steam acceptance of thehigh pressure turbine is reduced, whether as a result of closure ofcontrol valves, or reduction of rate of steam generation, but theproblem is not consequential if rate of steam acceptance, and hence alsoMSR pressure reduction, does not exceed a valve which depends, in part,on the way the MSR drain system is designed.

When control valves are held open, and intercept valves are fullyclosed, and thereafter fully opened, and especially if they are reopenedslowly, MSR pressure will at first increase, but, in due course, willdecrease to the value that applied prior to initiation of fast valving,and it has been the fact that the process of decrease could cause MSRdrain system instability that GE has warned against.

Pressure increase is greatest when intercept valves open slowly, andsince this fact is readily understood by those skilled in the art, itshould be, and in some quarters has been, obvious that speeding up theprocess of intercept valve reopening would minimize it, and it wouldpresumably be obvious, in turn, that such reduction of pressure risewould operate to minimize rate of subsequent MSR pressure drop.

However, I have conceived of additional techniques for reducing rate ofMSR pressure drop consequent on fast valving, which measures aredisclosed in this application.

Also, where there has been and remains a good deal of power industryconcern as to the MSR drain system problem, and a chance that it could,if indeed it does not already, stand in the way of employment of fastvalving of BWR and PWR nuclear installations, the fact that thetechniques hereinafter described for avoiding or largely minimizing rateof MSR pressure drop, as it would appear, have not heretofore beenadvocated, would .[.seam to be well indicated that they are notobvious..]. .Iadd.seem to indicate nonobviousness. .Iaddend.

G. PROVISION TO SELECTIVELY REPOSITION INTERCEPT VALVES OF LOW PRESSURETURBINES OF NUCLEAR STEAM ELECTRIC INSTALLATIONS, AS A MEANS OFEFFECTING SUSTAINED TYPE FAST VALVING FOR PURPOSES OF STABILITYIMPROVEMENT

This application introduces the concept of selectively effectingpreprogrammed processes of closing and reopening of intercept valveslocated ahead of low pressure turbines of nuclear type where more thanone low pressure turbine receives power from a single high pressureturbine, with the objective of improving power system stability whenjeopardized by suddenly occurring events.

Not only has there been no evidence of any prior art in this area, butthe concepts involved, when explained to turbine control people, havebeen viewed as novel.

SUMMARY OF THE INVENTION

The invention has relation to improved methods for rapidly varying thedriving power of turbines by repositioning intercept valves, andoptionally also simultaneously repositioning control valves andcontrolling the operation of steam supply systems, whereby to avoiddevelopment of power system instability when jeopardized by transmissionline faults and other stability endangering events, while at the sametime avoiding damage to equipment.

Generator drive systems of power system steam-electric installationscomprise a high pressure turbine, plus one or more low pressureturbines, plus, in the case of installations in which steam is generatedwith use of fossil fuel, or with an HTGR or high temperature gas cooledtype nuclear reactor, one or more intermediate pressure turbines whichare operated with steam that is highly superheated in reheaters.

Control valves are employed to control supply of steam to high pressureturbines and intercept valves are provided immediately ahead ofintermediate pressure turbines and low pressure turbines of nuclearinstallations which do not intensively reheat steam discharged from thehigh pressure turbine.

In the U.S., at least, it is common practice for the first stage of ahigh pressure turbine to be of the impulse type and for first stagenozzles to be grouped into segments with the steam supply to eachsegment individually controlled by means of individually operablecontrol valves.

It also was, at one time, a common practice and it remains feasible toemploy by-pass type control valves that admit high pressure steam tointermediate stages of high pressure turbines.

Providing to automatically fully or nearly fully close intercept valvesin response to an indication of a line fault, and thereafter fullyreopen, offers a way to decrease the tendency for power systemgenerators to lose synchronism as a result of line faults and othersystem stability endangering events, but momentary closure of this typetends to increase pressure within reheaters and moisture separators,with two effects,

1. the post-fault or more generally the post stability endangering eventdriving power of the turbine or turbines down stream of the interceptvalve or valves will exceed the pre-fault, or pre-event driving power, acircumstance which tends to adversely affect system stability, thisbeing especially the case when the fault or other stability endangeringevent results in the sustained opening of one or more transmissionsystem circuit breakers, and thereby operates to impede transmission ofpower in the post-fault or post-event regime, whereas actually it wouldnormally be advantageous for the total post-fault or post-event drivingpower of the turbine to be held less than, and, as a rule, preferablysomewhere in the range of 60 to 90 percent of pre-fault or pre-eventvalue,

2. reheat and MSR (moisture separator and moisture separator reheater)pressure safety valves may discharge steam and in some cases maythereafter leak and require maintainance.

One proposed approach to the solution of problem (2) above is to speedup the process of intercept valve reopening, while another is to raisethe setting of the reheater and MSR pressure safety valves.

However these approaches do not solve problem (1).

An obvious way to avoid both problem (1) and (2) is to repositioncontrol valves so as to reduce high pressure turbine steam acceptance ona sustained basis.

However, there has been reluctance on the part of engineers to employthis procedure because of the conviction that it would be difficult andexpensive to provide so that it could be effected without lifting ofhigh pressure safety valves, which, when occurring, is likely to causedamage to the valves that can require scheduling a unit shutdown toallow effecting repairs.

Also, in the case of fossil fuel fired installations, there has beenconcern as to the feasibility of readjusting fuel, water and combustionair supply rapidly and accurately enough to prevent damage to reheatersdue to overheating, and as to whether what would be done would causeobjection from the standpoint of excessive thermal fatigue damage toturbines.

Further, in the case of boiling water reactor, or BWR type nuclearinstallations, there are stringent limitations in respect to the extentto, and speed with which, control valves can be closed.

In the present invention the problems presented are dealt with byemploying a method of fast valving which brings into effectpreprogrammed process of,

1. intercept valve closure which are fast enough and sufficient inextent to have a favorable effect on generator rotor first swingstability, and preferably take the form of full closure effected in 1/4second or less,

2. subsequent reopening of some or all intercept valves to a partiallyopen position with reopening preferably initiated somewhat in advance ofthe first forward swing of the generator rotor, and carried tocompletion within a fraction of a second following the peak of thatswing,

coupled with

3. preprogrammed servo valve implemented retention of partially openedintercept valves in, or substantially in, the preprogrammed positionthat they attained in their rapidly executed reopening process,

with which techniques the intercept valves assume and retain a partiallyopened position at the end of a preprogrammed repositioning cycle, untilsuch time as an election is made to further reopen them.

In the case of turbines in which steam from a high pressure turbinepasses directly to two or more low pressure turbines the aboveprocedures may optionally be modified by either closing and holdingclosed the intercept valves of only one or more but not all low pressureturbines, or closing the intercept valves of all turbines and thereafterrapidly opening the intercept valves of one or more but not allturbines.

In one approach discharge of steam through reheat and low pressuresafety valves can be tolerated, which, if planned on, can be favorablyimplemented, on a control basis, by providing so that electricallycontrolled air operated lifters will be applied to a predeterminednumber of spring loaded type valves, and that those valves will belifted in response to a fast valving initiation signal.

In an alternate approach, which is consonant with practices thatcommonly apply in the case of continental European steam electricinstallations of fossil fuel type (35, 65, 66, 70, 71), opening ofreheat or low pressure safety valves is prevented by provision of servocontrolled by-pass systems that discharge desuperheated steam eitherfrom the hot reheat line, or, in the case of BWR type nuclearinstallations, from a point just ahead of the low pressure turbines tothe condenser, in response to increase in steam pressure.

In still another approach prevention of discharge of steam throughreheat or low pressure safety valves of PWR type nuclear or fossil fuelinstallations is prevented by employment of preprogrammed control valverepositioning.

Where discharge of steam through reheat or low pressure safety valves istolerated, as also where power operated reheat or low pressure steamby-pass systems are employed, provision to close control valves isoptional, while if control valve repositioning is employed election canbe made to employ any of the following preprogrammed procedures.

1. full or partial closure followed by full reopening,

2. full or partial closure followed by partial reopening,

3. partial closure.

From the standpoint of preservation of system stability, control valvefull closure in 1/4 second or less is advantageous, preferably followedby comparably fast full or partial reopening, with initiation ofreopening adjusted to take place so that full extent of preprogrammedreopening is achieved within a fraction of a second following completionof the first forward swing of the generator rotor.

Where boiling water reactors represent the steam supply source, itbecomes necessary to limit the speed of control valve closure and toavoid closure that, even momentarily, causes a reduction of steamacceptance of the high pressure turbine that exceeds the capacity of theturbine's high pressure by-pass system.

In steam electric installations in which, following continental Europeanpractice, desuperheating type steam by-pass systems are provided as away to automatically by-pass superheated steam around the high pressureturbine to the cold side of a reheater located ahead of an intermediatepressure turbine, control valves can be rapidly repositioned in anydesired degree, on a sustained basis up to a point dependent on thecapacity of the by-pass system, but would preferably be fully closed andthereafter partly reopened to a point at which the steam supplied to thecold reheat line coincides with the steam acceptance of the intermediatepressure turbine, at the time of completion of preprogrammed partialintercept valve reopening.

In the case where the steam supply source comprised a PWR, orpressurized water type reactor, there would normally be no restrictionon fast full closure of control valves, provided that they were promptlyreopened to a point at which reduction of high pressure turbine steamacceptance did not exceed the capacity of the turbine's high pressureby-pass system.

In the case of both PWR and BWR reactor type steam supply sources, itcan be advantageous to minimize both moisture separator reheaterdepressurization and extent of pressure rise, which implies, that, inthe case of PWR installations, it is important to closely relate theextent of preprogrammed reduction of steam acceptance of the highpressure turbine to that of the low pressure unit.

Implementation of intercept and control valve repositioning can takesome or all of several types of preprogrammed procedures listed below.

a. fully closing all intercept valves in a fraction of a second byrapidly opening valve actuator oil dump valves,

b. fully or partly closing all intercept valves under servo control,

c. after initial full or partial closure of all intercept valves,repositioning under servo control,

d. supplementing item (c) by fast partial intercept valve reopeninginitiated somewhat in advance of the generator rotor first forwardswing, preferably effected within 1/2 second, with control of extent ofreopening determined with use of metering cylinders or with servo or camoperated valves,

e. fully closing all or some control valves in a fraction of a second byrapidly opening valve actuator oil dump valves, or

f. fully or partly closing all control valves under servo control,

g. after initial full or partial closure of control valves, fully orpartly reopening under servo control

h. supplementing item (g) by fast full or partial control valvereopening initiated somewhat in advance of the instant of generatorrotor first forward swing, and preferably effected within 1/2 second.

Supplementary preprogrammed initiation of reduction of rate of steamgeneration and initiation of steam by-passing operations and ofdischarge of steam to atmosphere can also, optionally, be employed, andprovision of a supplementary preprogrammed process of full opening ofintercept valves can additionally be elected.

A main object of the invention is to allow, via provision in generatingstation design, so that power transmission lines can be subjected tohigher transmitted power loadings than could otherwise be employedwithout a consequent increase in hazard of development of systeminstability on the occurrence of line faults and certain other systemstability endangering events.

Another object is to increase the amount of power that can be safelytransmitted over a right of way of given width.

A further object of the invention is to allow increasing the amount ofpower that can be transferred over a line operated at a given voltage.

Another object of the invention is to provide so as to minimize hazardof development of system instability in the event of infrequentlyoccurring severe contingencies such as delay in fault clearance.

Another object of the invention is to achieve the above objectives in amanner that minimizes generating station first and operating costsincluding costs related to providing for steam bypassing and fordischarging steam to atmosphere via power operated valves, and thatavoids need to take generating units out of service to allow repairs.

Another object of the invention is to provide improvements in generatingstation design which increase effectiveness and eliminate or minimizepenalties in employment of fast turbine valving, whether or notsupplemented by employment of dynamic braking, as a way to preventcascading type system instability.

Another object of the invention is to avoid development of systeminstability subsequent to the occurrence of first generator swingsfollowing a line fault or some other system stability endangering event.

Another object of the invention is to avoid situations where, eventhough a generator remains in synchronism following a fault on a linetieing it to a system, the disturbance resulting from the fault has theeffect of causing loss of sychronism of some other generator orgenerators.

Another object of the invention is to effect fast turbine valving forsystem stability improvement in ways which do not necessitatereadjustment of rate of steam generation within steam generators.

Another object of the invention is to effect fast valving of steamelectric installations which receive their steam from either PWR or BWRnuclear steam supply sources, in such manner as to eliminate significantreduction of moisture separator reheater pressure, whereby to avoidinstability of MSR drain systems.

Still another object of the invention is to prevent the scramming of thereactor when provision for fast valving for stability improvementpurposes is made in BWR type nuclear steam electric installations.

It is an important element of the invention that it can be usefullyemployed as an aspect of a process of combined fast valving andmomentary application of braking load.

BRIEF DESCRIPTION OF DRAWINGS

The subject matter which is regarded as the invention is capable ofbeing implemented in a variety of ways. In practice what is necessary tofacilitate its employment is to devise ways to apply it in power systemsteam electric installations of already developed types, with a minimumneed to introduce changes in design that would be costly and timeconsuming to put into effect.

Therefore the drawings have been prepared in this context.

In the drawings

FIG. 1 is a simplified schematic view of a typical fossil fuel typesteam turbine driven generating unit of U.S. design to which is coupleda drum boiler type of steam generator,

FIG. 2 is a simplified schematic view of the No. 2 unit of TVA'sCumberland generating station in which a Brown Boveri cross compoundturbine receives its steam from a Babcock & Wilcox once through steamgenerator,

FIG. 3 is a simplified schematic view of a large nuclear turbine whichis supplied with steam from a nuclear steam supply source which could beof either the boiling water (BWR) or pressurized water (PWR) reactortype,

FIG. 4 is a simplified partial representation in schematic form of anarrangement adapted to rapidly reopen a closed turbine steam admissionvalve but do so part way only,

FIG. 5 represents an exterior view of an intercept valve that has beenprovided with means for effecting fast partial reopening,

FIG. 6 comprises FIG. 1, modified to include a desuperheating typeby-pass system that is arranged to discharge steam from the hot reheatline to the condenser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 water that has drained down to the bottom of condenser 1 ispumped by pump 2 through low pressure feed water heater system 3 todeaerator 4 from which point it flows to boiler feed pump 5 which isdriven by boiler feed pump turbine 6 which receives steam from one ormore sources not shown through valve system 7.

From the boiler feed pump water passes into high pressure feed waterheater system 8, next to economizer 9 and next into drum 10 from whichit passes to the bottom of and up through furnace 11 and returns to thedrum as wet steam.

The steam so produced next passes through superheater 12, stop valves 16shown for convenience as a single valve, but normally consisting of fourvalves one being series connected to each of the four control valves 17of partial admission type high pressure turbine 18, wherein the nozzlesahead of the control stage of the turbine are divided into four segmentswhich are supplied with steam individually through the four controlvalves.

After passing through the control valves and the turbine the steamenters reheater 19 and from there flows through a pair of stop valves 21and series connected control valves 22, and then into intermediatepressure turbine 23 and from there to low pressure turbine 24 and fromthere flows into the condenser where it is reconverted to water. Thethree turbines are coupled together in line and drive generator 26 whichsupplies three phase power to three transformers 31 through generatoroutput leads 29. In the transformer 31 the voltage is stepped up fromtypically around 22,000 volts to a voltage which is today typically inthe range of 345 to 500 kv, and can range up to 765 kv.

The transformer connects to switching station 32 through a pair ofcircuit breakers 37 and 38, shown conventionally as square boxes, whichare to be understood to represent two of a larger number of circuitbrakers not shown in the drawing by which the generator makes connectionto transmission lines L₁, L₂ and L₃ and, in the bulk of cases, also toat least one other generator located within the same station that housesthe generator shown.

Transmission system responsive control system 33 is to be understood toincorporate a protective relaying system which acts to cause the openingof circuit breakers at which lines terminate, on the occurrence of linefaults, and in the case of certain other events, and is to be understoodto incorporate also fast valving signal generating and logic means whichmay and usually would be made responsive to one or more parameters ofsystem prefault or, more generally pre-system disturbing event systemconditions, such as lines not in service and the magnitude of generatedand transmitted power, plus the fact of occurrence of a fault or otherevent of a type that could endanger system stability, which in the caseof a fault can depend on fault type and location, while also the controlsystem may be arranged to respond to the occurrence of a stuck breakeror some other post instant of fault initiation event, or to the extentand distribution of line fault induced reduction in power flow over oneor more lines, or in respect to the extent and rate of reduction of thepower output of generators, (57, 58).

Procedures of these types and others directed to determining when toinitiate and to modify fast valving cycles have already been describedin several patents (3, 21, 22, 23, 46, 54) one of which has alreadyexpired, while certain additional procedures are described in thewriter's pending patent application ser. no. 244,594.

In addition control system 33 usually receives information by carriercurrent or some other channel of communication which has relation topower flow over intra and inter system tie lines and as to the poweroutput of other generators located in and remote from the station, whichinformation is used to develop a signal that is created for the purposeof suitably modifying the load reference of the turbine's control systemso as to cause the turbine to become a participant in programs of tieline power flow control and system economic dispatch (53).

Also as shown by the dotted line connecting polyphase watt transducer 36which is connected to current transformers 34 and potential transformers35 and which generates a signal proportional to generator power output,control system 33 receives such a signal as one of its inputs.

Turbine and steam generator control system 39 receives as inputs theoutputs of generator rotor speed transducer 27, which usually appears inthe form of a frequency signal which is generated by a magnetic pick-upwhich is influenced by a toothed gear on the generator shaft, and inaddition receives as inputs an output from watt transducer 36 andoutputs from transmission system responsive control system 33 which takethe form of turbine governor load reference modification signalsgenerated as aspects of tie line and economic dispatch control systemsand also one or more types of signals which initiate fast valving, orthat may relate to what will be preprogrammed to be done when fastvalving is initiated.

Thus the dotted line that in FIG. 1 runs from transmission systemresponsive control system 33 to turbine and steam generator controlsystem 39 is to be understood to include as a minimum two channels ofinformation transfer namely one that is used to modify the turbine'sload reference system as an aspect of tie line and economic dispatchcontrol systems and at least one fast valving signal transfer channel.

However it is also to be understood that optionally, in addition, thenumber of information channels can be expanded to allow selectiveinitiation of more than one type of preprogrammed fast valving cycles,and to permit modification of the parameters of such programs inresponse to such factors as system conditions existing prior to a linefault or other system stability endangering event, and the occurrenceand nature of post fault events.

Coming now to the functions of turbine and steam generator controlsystem 39 these are in the first instance to continuously control theposition of the turbine's control valves 17 and also valve system 7 ofthe boiler feed pump turbine 6 and boiler fuel and air supply controlsystem 54, and in addition the position of control and intercept valvesin response to fast valving signals where the object is system stabilityimprovement and also as an aspect of turbine overspeed control systemswhich systems may also provide for control of stop valves 16 and 21though in the first instance these stop valves are controlled byemergency governors that represent a built-in feature of the turbine.

When it comes to providing to implement fast valving one useful thing todo that has not been provided for in U.S. steam electric installationsto date with exceptions in the case of Four Corners, and TVA's stationsCumberland and new stations comprises

1. effecting a process of sustained partial control valve closure with aview to avoiding development of instability on generator rotor secondand following swings and in the steady state following loss of one ormore lines, and as a way to avoid need to trip-off a generator such ascould otherwise apply.

Also another new but desirable thing to do is to

2. so provide that the curve of turbine driving power versus time beginsto rise at about the time that the generator rotor has attained the peakof its first forward swing so as to reduce the extent of generator rotorfirst backward and second and following forward swings (56).

Also a further thing that is desirable is to

3. avoid lifting high pressure safety valves 40 or 41 in order toprevent damage that could require scheduling a turbine shutdown for thepurpose of effecting reapairs.

Objectives (1) and (3) tend to be in conflict in that reduction of highpressure turbine steam .[.accepts.]. .Iadd.acceptance .Iaddend.such asoccurs when control valves close operates to cause increase in pressureahead of the turbine. .[.However provision.]. .Iadd.Provision.Iaddend.of one or more power operated relief valves 42 which often arearranged to make connection to the high pressure steam line ahead of theturbine stop valve through shut-off valves 44, and which are arranged toopen when a pressure operated switch in control unit 43 senses the factthat steam pressure in the steam line exceeds a preset value.Iadd., canprovide a solution. .Iaddend.

However, providing a sufficient number of these valves to preventlifting of valves 41 adds to the expense of the station, and especiallywhen, due to sluggishness of servo controlled valve repositioning, itwould be useful from a fast valving standpoint to at first fully closethereafter reposition control valves to a partially open position.

However there is a feasible way around this problem which can be viewedas obviously offering advantages once it is grasped but that has notgiven evidence of being at once obvious to those skilled in the art, orwho would profess to be skilled in the art of fast valving for systemstability improvement, namely to provide to close two out of the totalof four control valves that are commonly employed on partial admissiontype turbines, which brings with it the opportunity to rapidly reduceturbine steam acceptance to around 65 percent when starting from aninitial condition of full load.

Alternatively it can be elected to provide to close only one valve,which however reduces driving power only about 8 percent or thereabouts.

Broadly the concept is to rapidly close some but not all control valves,so as to take advantage of what is feasible when partial admission isused.

Apparently there would be objection on the part of turbine producers tothe rapid closure of more than half but less than all of the turbinescontrol valves so that in practice not more than half would be soclosed.

Also it is a feature of the present invention that, at the same timethat a pair of control valves, or only one valve, would be rapidly fullyclosed by valve actuator oil dumping, the load reference of the speedgoverning mechanism would be rapidly reset and the dump valves rapidlyreclosed so as to cause all valves to begin to move toward newpreprogrammed positions under servo implemented feed back type control.

How in detail the foregoing can be provided for can be well understoodby referring to U.S. Pat. No. 3,602,617 (54) which describes means forrapidly closing both control and intercept valves and for equallyrapidly modifying turbine load reference.

Thus, referring to FIG. 1 of the patent, it will be seen that ifprovision is made to replace the unbalance relay logic thereinidentified as item 9 by preprogrammed fast valving logic provided withinturbine and steam generator control system 39 and initiated in responseto a transmission system responsive control system 33 fast valvingsignal output, what is wanted will be fully accomplished if

a. the connections from the logic system to trigger 17 are opened in thecase of two (or three) control valves, and

b. the modifier is preset to bring about the desired sustained partialreduction in turbine load, rather than zero export load, such as thepatent stipulates.

Coincidentally with causing control valve repositioning the fast valvingsignal would be arranged to suitably modify boiler fuel and water supplyby temporarily disabling usual feed back controls and imposing a fastrunback type of control action which will have the effect of readjustingthe rate of fuel and feed water supply to new values that will beapproximately in balance with the preprogrammed new sustained value ofhigh pressure turbine steam acceptance in the post-fault or moregenerally in the post system stability endangering event regime.

It is not necessary to disclose in this application the details of howthis would be done because means of providing fast runback of fuel andfeed water supply, and hence steam generation, have for long beencommercially available from leading boiler and/or boiler controlproducers and at most would require some degree of speeding up. (61,62).

Since if valves 42 open quickly such opening will slow down pressurebuild up in the superheater it also applies that in the interest ofgetting maximum advantage out of each valve, and hence minimizing thenumber that would need to be provided to prevent lifting of highpressure safety valves 41, it can also be useful to provide, as viaenergization of a quick closing time delay reopening relay, so that thefast valving signal causes control units 43 to immediately open valves42 on a feed forward basis rather than in response to pressure rise, andretain them in open position for a period long enough for thepreprogrammed reductions in fuel and feed water supply to take fulleffect, which perhaps would require a minute or more.

In the U.S. up to now, except at Four Corners and in TVA's newerstations, only the simplest form of fast valving has been provided byturbine-generator manufacturers as a response to customers requests forprovision of fast valving as a means of system stability improvement,namely a system in which intercept valves only are repositionedmomentarily.

In the case of GE what has been offered has conformed to what is shownin the upper part of FIG. 6 of U.S. Pat. No. 3,601,607 (54) in whichinitiation of fast valving depends on the magnitude and rate of increaseof an unbalance between prefault turbine driving power and generatorelectrical load under fault conditions.

Actually response to this type of signal tends to be insufficientlyselective (59) and for this reason it can be useful to employ a fastvalving initiation signal provided by a transmission system responsivecontrol system as a permissive control that would supplement response togenerator power-load unbalance.

However in addition to providing for permissive control of fast valvingof the up to now usually provided type it will normally be advantageousto preprogram at least some degree of fast reduction in high pressureturbine steam acceptance plus a related fast runback of boiler fuel andfeed water supply partly as a way to prevent lifting of reheat pressuresafety valves and partly for reasons of system stability improvement.

Where the fault condition occurs on a radial line or on a weak tie toother systems, control system 33 can recognize this fact, as also theprefault load on the line and from this information, if warranted,generate a fast valving signal that calls for only a small partialsustained or perhaps no sustained reduction in turbine driving power,and perhaps for fast full closure of only one control valve by valveactuator oil dumping, while, if a fault occurs on a strong tie that iscarrying a heavy load, system 33 can recognize this condition andgenerate a signal that calls for a rapid closure of two or even allcontrol valves by means of dump valve action (59).

Coming now to providing for fast partial reopening of intercept valves,as a first step it is necessary to provide so that intercept valveactuator oil dump valves reclose before reopening can be started, andsince it is desirable for intercept valves to begin to open somewhat inadvance of the first forward swing of the generator rotor, (50) andsince time is required in which to bring about valve acceleration in areopening direction, it works out that in situations where generatingstations are interconnected by short lines of extra high voltage, thatit can be desirable for dump valves to reclose in as little as 0.05 to0.10 seconds following intercept valve closure.

Present GE dump valves which conform in design to what is shown in U.S.Pat. No. 3,495,501 (55) and are spring loaded to close, do not recloseuntil almost a second after the intercept valve closes.

However Westinghouse dump valves which are power operated to reclose doso as rapidly as required, and dump type valves also are commerciallyavailable that are equally fast.

Therefore there is nothing to prevent GE from providing sufficientrapidly acting dump valve means.

Since Westinghouse usually does not control its intercept valveactuators with servo control, and since GE's servo control is slowacting, to achieve the objective of rapidly implemented partialreopening in addition to providing ro rapidly reclose intercept valveactuator oil dump valves, it is necessary to provide via oilaccumulators so that oil needed to reopen the valves can be suppliedrapidly enough to cause them to open with sufficient speed, and alsoprovide so that the process of rapid opening terminates when the valvesreopen only part way, as say when they are 25 to 50 or perhaps 60percent open (50).

In the matter of limiting the extent of high speed reopening, oneapproach would be to provide to admit oil to the valve actuatorcylinders through position operated tapered spool decelerating typevalves that would be arranged to close in response to cam action as theintercept valve opens.

In another and perhaps simpler approach a metering cylinder can beinterposed between the valve actuator and the accomulator.

FIG. 4 shows a modification of the valve actuator mechanism shown inFIG. 2 of U.S. Pat. No. 3,495,501 which includes a metering cylinder 71which when forced down by admission of oil at the rod end will cause oilto flow into valve actuator cylinder 70 and push its piston upward. Asshown in the figure to avoid undesirable impact effects the piston ofthe cylinder is provided at the bottom with the same type ofdecelerating device, taking the form of a tapered spear protruding fromthe bottom of the piston, that is provided at the bottom of the actuatorpiston.

In FIG. 4, the piston of the metering cylinder is shown at mid strokewhile for ease of inclusion in the diagram the cylinder has been shownmounted so that its rod end faces upward.

Actually it would appear to be preferable, however, to mount thecylinder with the rod end down as shown in exterior view in FIG. 5wherein a pilot operated normally closed two way valve 72 which iselectrically opened by energization of electrically controlled valve 73provides a way by which oil stored in accumulator 74 can cause thepiston of metering cylinder 71 to rapidly stroke upward therebyeffecting rapid lifting of the piston of valve actuator cylinder 70.

Referring further to FIG. 5, 75 is a check valve which serves as a pointat which oil can enter the accumulator from the oil supply system while76 is an adaptor that provides for connection of valve 72 to themetering cylinder and that is provided with a bleed connection to adrain.

Item 77 represents the slow reclosing dump valve shown as item 10 inU.S. Pat. No. 3,495,501, while item 78 represents a duplicate of valve72 which can function as an auxiliary fast reclosing dump valve since itis arranged to by-pass oil around th piston of cylinder 70, and isactivated to open by energization of electrically controlled valve 79.

By reference to FIG. 1 of U.S. Pat. No. 3,495,501 it will be noted thatin FIG. 5 the intercept valve assembly is being viewed from that side atwhich steam enters the valve, which is the reason why oil return line 80of FIG. 4, which is item 9 of FIG. 2 of U.S. Pat. No. 3,495,501, is notvisible.

The concept behind the showing of an auxiliary dump valve is that if itdid not turn out to be easy to modify the valve described in U.S. Pat.No. 3,495,501 so as to render it fast reclosing, this valve could stillbe retained in use as a way to provide overspeed protection, whileemployment of fast reclosing as well as fast opening dump valve 78 wouldbe effected primarily as a means of implementing fast valving, though itcould also be used to provide a redundant means of closing the interceptvalve in response to a condition of overspeed.

In the writers concept valves 72 and 78 could comprise a commerciallyavailable very rapidly acting valve that has been widely used forcontrolling the operation of die casting machinery.

However despite its record of successful use the turbine producer couldwell prefer to use his own time tried valve as a way to perform the veryimportant function of protection against overspeed, which being thecase, the provision of an auxiliary dump valve for control of fastvalving which could also function as a redundant means of initiatingvalve closure in response to overspeed conditions would serve thepurposes of fast valving yet could in no way serve to degradereliability of overspeed protection.

Returning now to FIG. 5, control system 81 is arranged to control theposition of valves 73, 77 and 79 through electrical connections shown asdashed lines.

In service use the accumulator containing its normal complement of oiland the piston of the metering cylinder 71 is up against the rod end ofthe cylinder so that the piston rod is fully extended downward. Alsoelectrically controlled valves 73 and 79 are deenergized and valves 72and 78 closed.

It is provided that when an event occurs that results in a fast valvingsignal input to control system 81, valve 79 is energized, which causesvalve 78 to open with the effect that the intercept valve closes.

After alowing time for closure to take place, and optionally alsoensuring that it did take place by means of a feeler switch which is notshown, the control system energizes valve 73 which causes valve 72 toopen whereupon the piston of metering cylinder 71 strokes upward andforces the piston of cylinder 70 to rise part way.

Next after a delay period valve 73 is deenergized which causes valve 72to close and because oil can slowly drain out of adaptor 76 via itsdrain connection, the piston of the metering cylinder drops down at arate governed by the rate of discharge via the drain, which is made lowenough so that there is no problem of impact when the piston comes torest at the end of its stroke, at which stage the valving cycle iscomplete.

One incidental but not unimportant advantage that the metering cylinderoffers relative to decelerating type valve, that may be worthy of note,is that by providing a supplementary push button control that would actto energize electrically controlled valve 73 the metering cylinder canbe from time to time stroked under normal load conditions, and byproviding the accumulator with a pressure gauge and observing thepressure drop when stroking takes place it could be easily determinedwhether or not the accumulator contains its normal content of nitrogenand if it did not, provide to add nitrogen.

Because electrically controlled valves 73 and 79 need to be fast acting,use of an alternating current type of solenoid valve would offeradvantages, which however would be in part offset by the need to supplypower to these valves by means of an inverter which would take its powerfrom a storage battery. One solution to this problem would be to employa dc solenoid valve having laminated magnetic components as a way toavoid eddy currents which develop in solid type solenoid plungers andslow down valve operation.

Where intercept valves are of plug type as is customary in fossil fueltype turbines it works out that typically 8 to 10 percent stroke willopen the valve enough to pass 35 percent of full load steam with reheatpressure at the value that applies at full load, and that around 13 to15 percent will supply 60 percent, which means that only a relativelysmall volume of oil is needed when fast partial valve opening isplanned, which implies in turn need to employ only a short meteringcylinder and a relatively small accumulator. Also because of the smallamount of valve stroking needed, where the point applies that GEintercept valves are provided with servo valves, use of a meteringcylinder could be dispensed with if servo controlled stroking rate wereincreased from their present usual value of 10 seconds full stroke toaround 21/2 seconds full stroke.

Where intercept valves are not provided with means of servo control themetering cylinder approach would appear to provide a relatively simplesolution to the problem of limiting extent of high speed reopening.

So far what has been said on the subject of control valve operation hashad relation to U.S. units having partial admission type high pressureturbines which also typically do not have provision for rapid enoughstroking of valves under servo control to serve effectively as a way tobring about a reduction of high pressure turbine steam acceptance thatwill serve the needs of fast valving.

In cases where high pressure turbines are not equipped for partialadmission and provide fast enough control valve stroking under servocontrol to sufficiently limit turbine speed under entire loss of load,as applies in the case of Brown Boveri units, rate of valve closure,when fast valving is initiated for purposes of system stabilityimprovement, has turned out to be not too low to afford stabilityimprovement based on the fact that the Brown Boveri units in questionhave had higher specific inertias then steam-electric units of U.S.manufacture.

With servo control available it is possible to fully or nearly fullyclose control valves and completely or nearly completely close interceptvalves, and to also thereafter reopen both types of valves part way,provided that oil accumulators are made use of as a way to ensuresufficiently rapid valve reopening.

This method of accomplishing fast valving has been provided for in thecase of unit No. 2 of TVA's Cumberland station, in which a 1300 MW crosscompound Brown Boveri turbine is supplied with steam by a Babcock &Wilcox oncethrough boiler, the general arrangement being as shown inFIG. 2 wherein like identifying numbers have like meanings toidentifying numbers of FIG. 1.

Numbers not shown in FIG. 1 comprise primary and secondage superheaters14 and 15, fly ball type turbine speed sensor 28, superheater by-passvalve 47, with is control unit 48, flash tank 49 and valves 50 which arearranged to open in response to an excess of flash tank pressure.

From the standpoint of fast valving the important feature shown in FIG.2 is the nature of the superheater by-pass valves provided as an elementof the steam generator, which comprises an array of fast acting airoperated valves which, when opened, allow steam to flow to the condenservia the flash tank, and which, taken together, have proved to haveenough flow capacity to prevent lifting of high pressure safety valveseven in the event of a turbine trip-off taking place at full load. (37).

The fact that these valves both offer this much steam acceptancecapability, plus the fact that, unlike superheater by-pass valvesprovided by the two other leading U.S. producers of the power stationboilers, they are fast acting, implies that there is no objection toemploying full closing of all control valves, and thereafter reopeningpart way under servo control over a period of up to 10 seconds whichrepresents the time required for control valves of GE electrohydraulicturbine control units to reopen full stroke.

This built in by-pass capability not only affords something in the wayof an advantage as regards opportunity for system stability improvement,but, more important, eliminates need to purchase and install poweroperated relief valves at added cost where provision for fast valving isbeing made.

When it comes to how to provide so that B & W's superheater by-passvalves are caused to open .[.when fast valving is involved,.]. when fastvalving is invoked at Cumberland, B & W's initial approach will be toprovide so that they open without delay in response to development of apredetermined increase in pressure within the superheater system andreclose progressively as pressure falls.

An alternate approach would be to preprogram a process of valve openingthat would be designed to prevent a rise in, or to somewhat reducepressure, and that would be followed by a process of progressive valvereclosing as pressure dropped below a preset value.

Referring now to FIG. 3, which represents a nuclear steam-electricinstallation, steam supplied by nuclear steam supply souce (NSSS) 13which could be of either the pressurized water reactor (PWR) or boilingwater reactor (BWR) type flows principally into high pressure turbine 18while some is diverted to he steam reheat coils located within moistureseparator reheater (MSR) 20.

In the figure there is a line from the moisture separator reheater whichdrains to drain tank 60, from which drain water flows normally throughcheck vlve 61 and drain tank level responsive valve 63 into the lowpressure feed water system 3, but can also flow to the condenser throughcheck valve 62 and drain tank level responsive valve 64.

Whereas only one MSR, and only one drain tank 60 and associated valving61 through 64 is shown, it is to be understood that in actuality thereare two MSRs each with its own drain tank and set of associated valvesfor each low pressure turbine, or in the installation shown in FIG. 3 atotal of 6 MSRs, 6 drain tanks and 6 sets of valves.

Steam that passes through the MSRs enters 3 low pressure turbines 24 via6 pairs of stop and intercept valves 21 and 22 respectively.

In PWRs item 40 represents a high pressure safety valve that dischargessteam to atmosphere while in BWRs it represents a safety valve thatdischarges direct to the suppression chamber of the reactor or thecondenser.

Similarly, in the case of PWRs safety valves 45 and 46 are arranged todischarge low pressure steam to atmosphere, and in the case of BWRs tothe condenser.

Items 50 represent groups of by-pass valves that are arranged to open inresponse to excess steam pressure ahead of the turbine, such as candevelop when the steam acceptance of high pressure turbine 18 is reducedby closure of control valves 17.

In the case of PWR reactors of Westinghouse type at full load steamdelivery pressure falls well below pressure at no load and it resultsthat a sufficiently brief momentary full closure of control valves 17plus a sustained 50 percent reduction in high pressure turbine steamacceptance will not lift safety valves 40.

On the other hand it is to be understood that if the NSSS is of BWRtype, the by-pass capability of valves 50 limits, to the capacity of theby-pass system, the extent of even only momentary reduction in highpressure turbine steam acceptance that can be tolerated withoutscramming the reactor.

For the above reasons and because by-pass capability is expensive, inthe case of those BWRs which do not have 100 percent by-pass capability,which is the usual situation, and assuming partial admission units areinvolved, it can be essential to rapidly fully close no more than twoand in some cases only one control valve.

In the case of nuclear turbines Westinghouse units employ butterfly typeintercept valves which have the advantage that in closing they operateto very rapidly reduce low pressure turbine steam acceptance, but thedisadvantage that when opened conventionally at a steady rate over aperiod of 5 seconds reacceptance of steam by the low pressure turbine isdelayed for over two seconds which is disadvantageous and therefore itis important to provide via fast closing dump valves, accumulators andmetering cylinders or perhaps cam operated decelerating valves so thatthe valves rapidly reopen part way, as in the range 25 to 50 percent ona flow basis within 1/2 second after the peak of the generator rotorfirst forward swing.

How this could be accomplished would differ in detail only from what isshown in FIGS. 4 and 5.

Whereas in the case of both fossil fuel and nuclear steam turbines thedesirability of making provision for fast partial reopening of interceptvalves has been stressed it could also apply that providing for fastpartial reopening of control valves could prove advantageous insituations where it might serve to limit requirements as to need foradditional steam by-pass capability.

In the area of problems that could arise in application of fast turbinevalving to nuclear steam electric installations the GE has cautionedthat fast valving, even of the type that employs only momentaryintercept valve closure, could give rise to difficulties in the way ofmalfunction of moisture separator reheater drain systems due to the mildform of MSR depressurization that takes place when intercept valvesreopen after at first initially closing.

To the extent that such a problem exists it would tend to be intensifiedwhen control valves are rapidly closed.

However, there is evidence which suggests that, with proper design ofMSRs and their drain systems, rapid depressurization has not and in thecase of fast valving wll not cause a problem of consequence.

Tests will be needed to clarify this point.

If, following tests, a problem remained that could not be readily solvedone solution would be to provide to fully close both the turbine'scontrol and intercept valves and after closure rapidly open them both toa point at which the control valve has reached its preprogrammed newsustained position and the intercept valve has reached an equally openposition on a flow basis, and providing thereafter to only slowly fullyreopen the intercept valves under servo and/or rate of oil flow control,while in the case of PWR type reactors or at any rate in the case ofWestinghouse PWRs this would not involve a need to provide added steamby-pass capability.

On the other hand it would represent a costly approach where BWRreactors were planned for use because it would require providing onehundred percent by-pass capability.

However in the case of BWRs, and for that matter also in the case ofPWRs, an alternate approach appears to be feasible, due to the fact thatit is claimed that experience to .[.data.]. .Iadd.date .Iaddend.hasshown that, presumably due to the cleanliness of the steam and its lowdischarge velocity, low pressure safety valves of nuclear installationshave not leaked following discharge of steam, whether or not they are ofthe pilot operated type employing teflon O-rings which are widelyemployed in Westinghouse PWR installations, or of the spring loaded typeused by GE in BWR and also in PWR installations.

To the extent that this claim can be relied on as a guide to the future,the point would apply that it is feasible to control turbine drivingpower in the period following the generator rotor first forward swing,by merely providing to suitably control intercept valve reopening (52)during the entire period during which steam generation within thereactor is being reduced, and rely on discharge of steam through lowpressure spring loaded safety valves to limit rise in MSR pressure.

Moreover by providing to lift these valves in response to activation ofelectrically controlled air operated lift cylinders (51) with the use ofpressure switches which could be preprogrammed to provide control onlywhen fast valving has been .[.involved.]. .Iadd.invoked.Iaddend., thevalves could be employed as a way to hold MSR pressure constant duringthe entire fast valving process, thereby avoiding need for concern as tothe behavior of MSR drain systems.

Furthermore it might also prove feasible to extend this concept tofossil fuel installations.

In the fossil fuel case the point would apply that experience has shownthat reheat pressure safety valves are less likely to be damaged bydischarge of steam than are high pressure types, due presumably to thelower velocity on steam discharge.

Also there is reason to believe that providing to lift safety valveswith an air cylinder, rather than merely allowing them to lift on theirown in response to increase in steam pressure, also can be expected tominimize damage effects.

Therefore, and especially if steps are taken so that the boiler,superheater and reheater are kept in a clean condition (63) the approachof providing for control of driving power in the period following thegenerator rotor first forward swing via control of rate of.Iadd.intercept valve .Iaddend.reopening could represent a workableprocedure.

When it comes to how to regulate intercept valve reopening, there wouldremain the desirability of first rapidly opening the valves part way,and then proceeding more slowly.

When it comes to control of position in the period following initialfast partial reopening, the point applies that it is well within theskill of control system designers to provide, as with the aid of flowcontrol devices, and/or servo systems which could be equipped with atime varying control input that could comprise a motor driven cam thatvaried the position of a core in a linear differential transformer, soas to effect preprogrammed processes of intercept valve reopening, suchthat following an initial rapid drop during the period of generatorrotor first forward swing, turbine driving power would be restored to anew preprogrammed sustained value, which in the case of fossil fuelinstallations would preferably be selected to be somewhere in the rangeof 60 to 90 percent of full load value, but in the case of PWR and BWRnuclear installations, could cover a wider range, since thermal fatigueeffects represent a minor factor in the life of nuclear turbines ofthese types, due to low value of steam temperature.

One point that has so far not been touched on relates to the fact thatit is not unusual for steam driven boiler feed pumps to receive theirsteam from an extraction point of an intermediate pressure turbine, inwhich case the turbine steam supply from this source is downstream ofthe intercept valves and will be much reduced, if it does notmomentarily disappear, when intercept valves are rapidly fully closed asan aspect of fast turbine valving.

This will result in a process of slowing down of the turbine which willoperate to reduce rate of feed water supply more rapidly than thepreprogrammed extent of reduction of heat release within the steamgenerator, but the speed with which this occurs will be governed by thecombined specific inertia of the turbine and pump, and, especially ifintercept valves are rapidly reopened part way, it has so far appearedto experts in the design of fossil fuel steam generators, that themomentary slowing down that would be experienced would not beconsequential as regards effect on the steam generator.

Moreover, in any case, turbines that, at over a predetermined load,accept steam from a point downstream of the intercept valve commonly areprovided with means to accept steam either or both from the cold side ofthe reheaters or the high pressure steam header at light loads.

Normally separate steam chests are provided as a way to allow transferto one or other of these steam sources and it could readily be provided,and may prove desirable, to effect transfer as a preprogrammed rapidlyexecuted step that would be put into effect in response to a fastvalving signal.

Similarly, if in the case of nuclear units, in some cases, boiler feedpump turbines draw steam from a point downstream of the intercept valve,provision can be made to rapidly transfer to the main high pressuresteam supply in response to a fast valving signal.

It is believed that the foregoing has served the purpose of showing howit is feasible to preprogram fast valving procedures, involvingsustained step reductions in turbine driving power which will well servethe purposes of power system designers when it comes to providing waysto minimize generation station first cost through avoiding need toinstall redundant circuit breakers, and also as a way to avoid need toconstruct redundant lines (36).

However to complete the picture it is necessary to provide so thatprocesses of diversion of steam to atmosphere, or to the condenser, thatneed to be employed as a way to prevent discharge of steam through highpressure safety valves will be terminated without too long a delay.

Actually this is easy enough to accomplish by merely providing tosimultaneously rapidly reduce heat release within, and feedwater supplyto, the steam generator on a preprogrammed basis, with provision totemporarily override normally utilized feed back type control systems.

Also as matters stand providing this type of control is already wellwithin the skill of designers of steam generator control systems,whether of types that are used in fossil fuel or nuclear steamgenerators. Thus systems for effecting fossil fuel steam generatorrunback, to the extent of 50 percent, accomplished in a matter of 30seconds (61, 62), have been provided by steam generator producers, tohandle contingencies such, for example, as a suddenly occurring outageof one of two parallel operating steam turbine driven feed water pumps,while it also appears that, even in coal fired fossil fuelinstallations, speeding up the process can be carried out so as toprovide a 40 percent runback in 10 seconds, although attaining thisspeed apparently has not proved to be critically needed as a way toavoid development of excess temperature of superheater components (62),and provision for 25 percent runback of BWR nuclear units in a matter of25 to 50 seconds, and of PWR units in 2 to 4 minutes, is typicallyfeasible.

Based on the foregoing the essential feature of the present invention isviewed as comprising an explanation of how it is possible to provide torapidly bring into effect preprogrammed control processes directed toeffecting sustained partial as well as momentary reduction of turbinedriving power, and do so with the use of techniques and equipment thatare essentially already available, except to the extent that certainminor changes in equipment for controlling the rapid positioning ofturbine valves represent features that are necessary to realization offull potentialities.

Moreover it is easily possible and will generally be useful to provide,within turbine and steam generator control system 39, a plurality ofpreprogrammed matched turbine and steam generator control processes, andto further provide so that, when an event occurs that sufficientlyendangers system stability to require initiation of fast valving,generating station system responsive control system 33 will not onlyinitiate it but will perform, in a preprogrammed way, the function ofselecting for initiation one particular pair of control processes fromamong the available plurality of matched pairs, as for example bysending to control system 39 an input that causes initiation of asustained reduction of driving power of 10 percent when a fault occurson line 1, but perhaps one of 20 percent if on line 2, and perhaps oneof 40 percent if, as evaluated by what is shown in U.S. Pat. No.3,657,552, it is expected that both lines will open due to delay infault clearance, or if one line is already open and the other open, andperhaps also initiate a 40 percent reduction when a fault occurs on line3.

Also it is possible to provide as per what is shown in U.S. Pat. No.26,571, so that in case of unsuccessful reclosure on a faulted line, theinitially selected pair of control processes are modified in apreprogrammed way, or so that the initially selected pair is modified ifreclosure is successful.

TABLE OF REFERENCES

1. S. A. Staege, "Regulator system," U.S. Pat. No. 1,705,688, Issue dateMar. 19, 1929.

2. Buell, R. C., et al, "Governor Performance During SystemDisturbances," Transactions AIEE, March 1931, Vol. 50, pp. 354-369.

3. S. B. Griscom et al, "Regulator systems," U.S. Pat. No. 1,935,292,Issue date Nov. 14, 1933.

4. S. B. Crary, "Power system stability - volume 11 transientstability," John Wiley & Sons, Inc., New York, pp. 194-197, 1947.

5. Mayer, "Fault initiated control of steam turbines as a means ofincreasing stability of power systems," Elektrichestvo, No. 13 - 1934,pp. 27-32.

6. Zdanov, "Stability of Electric Power Systems," Gosenergoisdat, 1938,pp. 293-310.

7. V. M. Gornshtein, "Improving the stability of power systems with weakties by acting on the regulation of steam turbines," Elektrichestvo, No.5 - 1955, pp. 27-31.

8. Murganov, B. P., "Experimental investigation of regulation ofturbine," Teploenergetica, No. 4 - 1957, pp. 9-15.

9. Murganov, B. P., Teploenergetica, No. 6 - 1959.

10. Murganov, B. P., "Regulation of power of turbogenerators in powersystems," Teploenergetika, No. 2 - 1961, pp. 9-13.

11. Kashtelan et al, "Response efficiency of excitation systems and theconditions for automatic voltage regulation of large turbo-generators,"Elektrichestvo, No. 10 - 1963, pp. 22-31.

12. N. I. Sokolov et al, "An analogue computer study of large turbogenerators in parallel operation," Elektrichestvo, No. 10 - 1963, pp.5-13.

13. V. E. Kashtelan et al, "Increasing stability of electrical systemswith help of fast regulation of steam turbines," Elektrichestvo, No. 4 -1965, pp. 1-8.

14. A. V. Shcheglyaev et al, "Some problems of using steam dumpingdevices in a steam-turbine unit," Thermal Engineering, vol. 12, No. 1,1965, pp. 1-9.

15. Y. F. Kosyak et al, "Initial experience of starting and running theprototype KhTGZ K-300-240 Turbine," Thermal Engineering, vol. 12, No.11, 1965, pp. 1-3.

16. Shubenko-Shubin et al, "The use of desuperheaters in aboiler-turbine unit," Thermal Engineering, 1967, pp. 28-32.

17. V. A. Venikov et al, "Use of fast acting governor control ofturbines as a way of improving power system stability," Elektrichestvo,No. 2 - 1967, pp. 13-21.

18. B. P. Moorganov, U.S. Pat. No. 3,421,014, Jan. 7, 1969, "Apparatusfor controlling operation of turbogenerator under emergency conditionsin the power system."

19. M. A. Berkovich, et al, "Automation for preventing system faults inpower pools," paper 34-06, 1970 Session International Conference onLarge High Tension Electric Systems (CIGRE).

20. G. A. Doroshenko et al, "Anti-disturbance automation devices forimproving power system stability," paper 34-05, 1972 SessionInternational Conference on Large High Tension Electric Systems (CIGRE).

21. R. H. Park, U.S. Pat. No. 3,051,842, Aug. 28, 1962, "Means formaintaining stability of power transmission systems during a fault."

22. R. H. Park, U.S. Pat. No. 3,234,397, Feb. 8, 1966, "Means formaintaining stability of power transmission systems."

23. R. H. Park, U.S. Pat. No. 26,571, of U.S. Pat. No. 3,234,397mentioned in item 22.

24. F. P. De Mello et al, "Turbine Energy Controls Aid in Power SystemPerformance," Proceedings of the American Power Conference, VolumeXXVIII, 1966, pp. 438-445.

25. R. G. Farmer et al, "Four Corners Project Stability Studies," IEEEConference Paper No. 68 CP 708-PWR, presented at San Francisco, Sept.15, 1968.

26. Philip G. Brown et al, "Effects of Excitation, Turbine EnergyControl, and Transmission on Transient Stability", IEEE Paper No. 70 TP203-PWR, presented at IEEE Winter Power Meeting, New York, Jan. 25,1970.

27. D. J. Aanstad, "Dynamic response and data constants for large steamturbines," IEEE Tutorial Course, Course Text 70 M 29 - PWR, pp. 40-49.

28. W. A. Morgan et al, "Modern stability aids for Calvert CliffsUnits," IEEE Transactions on Power Apparatus and Systems, Paper No. 70TP 147-PWR, Vo. Pas-90, No. 1, Jan/Feb 1971, pp. 1-10.

29. C. Concordia and P. G. Brown, "Effects of trends in large steamturbine driven generator parameters on power system stability," IEEEPaper No. 71 TP 74-PWR, pp. 2211-2218.

30. H. E. Lokay and P. O. Thoits, "Effects of future turbine-generatorcharacteristics on transient stability," IEEE Transactions on PowerApparatus and Systems, Vol. 90/1971, Paper 71 TP 75-PWR, pp. 2427-2435.

31. R. H. Park, U.S. Pat. No. 3,515,893, June 2, 1970, "Method ofimproving the stability of interconnected power systems."

32. R. H. Park, U.S. Pat. No. 27,842

33. R. H. Park, "Improved reliability of bulk power supply by fast loadcontrol," presented at American Power Conference Apr. 24, 1968.

34. cf Ref. 31 - column 19, para. 2

35. W. Trassl, "Safe cycling of high-pressure steam turbines, "Proc.American Power Conf., vol. 31, pp. 306-313, 1969.

36. E. Floyd Thomas et al, "Preliminary operation of TVA's CumberlandSteam Plant," presented at American Power Conference, Chicago, May 1973.

37. O. W. Durrant and R. P. Siegried, "Operation and control ofonce-through boilers during electric power system emergencies,"presented to IEEE Section Meeting, Dallas, Texas, Oct. 21, 1969.

38. Reference 50 of reference 33.

39. P. J. Martin and Ludwig E. Holly, "Bypass stations for bettercoordination between steam turbine and steam generator operation," Am.Power Con. 5/8/73.

40. Cushing et al, "Fast valving as an aid to power system transientstability and prompt resynchronization and rapid reload after full loadrejection," IEEE Paper 71 TP 705-PWR.

41. Reference 24, p. 442, column 1, para. 1.

42. Reference 29, p. 2211, column 2, para. 4.

43. Reference 27, p. 42, column 2, para. 2.

44. R. H. Park, "Fast turbine valving," paper T72 635-1, IEEE Trans onPower Apparatus & Systems, Vol 92 pp. 1065-73

45. Reference 44, p. 1069, column 1.

46. R. H. Park, U.S. Pat. No. 3,657,552, Apr. 18, 1972.

47. R. H. Park, discussion of reference 29, p. 2217, column 1, para. 5.

48. R. H. Park, discussion of reference 40, p. 1635, column 2, para 8.

49. A. C. Sullivan and F. J. Evans, "Some model experiments in fastvalving to improve transient stability," IEEE Paper No. C. 72 242-1.

50. Reference 44, p. 1067, column 1, para. 2.

51. Reference 44, p. 1066, column 2, para. 9 and 10.

52. Reference 31, column 19, para. 6.

53. Leon K. Kirchmayer, "Economic Control of Interconnected Systems,"John Wiley & Sons, Inc., Publishers, New York, 1959.

54. F. P. De Mello et al, U.S. Pat. No. 3,601,617, Aug. 24, 1971.

55. J. Kure-Jensen, U.S. Pat. No. 3,495,501, Feb. 17, 1970.

56. Reference 44, p. 1066, column 2, para. 3.

57. Reference 44, p. 1068, column 1, para. 11, through column 2, para.2.

58. Reference 44, p. 1067, column 2, paras. 3, 4, 5.

59. Reference 44, p. 1071, column 2, H. R. Stewart discussion ofreference 44, and p. 1071-1073, R. H. Park response.

60. Reference 44, p. 1067, column 1, para. 9 and 10.

61. O. W. Durrant, "Operation and control of once-through boilers duringelectric power system emergencies," 1970 Proceedings of the ISA, pp.1-14.

62. F. H. Fenton, Jr., and J. V. Pigford, "Rapid response andmaneuverability are obtainable from supercritical plants," 1970Proceedings of the ISA, pp. 15-26.

63. Reference 44, p. 1069, column 2, para. 3.

64. R. H. Park, "Relay and Control Techniques Used to Activate FastSteam Turbine Valving for System Stability Improvement" Minutes ofMeeting of Relay Committee, May 23-4, 1974, Engineering Section,Pennsylvania Electric Association.

65. K. H. Bieber, "Assured Power Supply With Modern Flexible GeneratingUnits and Bypass Systems Operating At Variable Pressure in A West GermanUtility System," IEEE Paper No. 71 CP 708-PWR.

66. CIGRE Committee Report, "The Electro-Hydraulic Governing Of LargeSteam Turbines," ELECTRA, No. 33, pp. 91, 114.

67. A. J. Smith and George Platt, "A Method for CorrectingTurbine-Generator Sudden Load Loss," Instrument Society of America,"Instrumentation in the Power Industry," Vol 14, 1971.

68. R. H. Park, U.S. Pat. No. 3,849,666, Nov. 19, 1974.

Further references cited herein comprise

69. W. Goodbrand and E. D. Holdup, "Recent load rejection testing oflarge steam generator-turbine generator units and an analysis of a majordisturbance on the Ontario Hydro system," presented at the IEEE-ASMEJoint Power Generator Conf. Sept. 1971.

70. "Sulzer safety systems for steam generators", Sulzer News no. 1/73,a publication of Sulzer Bros. Boiler and Nuclear Dept., Winterthur,Switzerland.

71. "Steam Conversion in Steam Distributing Systems" Braunkohle, Warmeand Energie, Vol. 12, No. 9, Sept. 1960, pp. 438 to 444.

72. Reference 44, page 1069, material under heading "Sudden Partial Lossof Area Export Load."

Referring now to FIG. 6, there is shown therein the steam electricinstallation of FIG. 1, modified by the inclusion of a by-pass systemwhich is adapted to discharge steam from the hot reheat line 81 tocondenser 1, and assumed, also, to incorporate provision for servocontrol of the position of the turbine's intercept valves, which valves,also, are further assumed to be of a type that is adapted to withstandlong duration service in a partially open position.

The by-pass system consists of one or more valves 82 which connect toone or more desuperheaters 83 which in turn connect to the condenserthrough one or more steam lines 84.

Valve 85 controls supply of water to the desuperheater, received viawater line 86, which, as shown, makes connection to the water line thatjoins boiler feed pump 5 to high pressure feed water heaters 8. Howeverthe use of an alternate source of desuperheating water is not excluded.

By-pass control system 87, and desuperheating water flow control unit88, can comprise equipment of types commonly used in ContinentalEuropean once through boiler steam electric installations (70,71).

As explained in reference 70, the control system of valves of this typeas produced by Sulzer Brothers, when used in fossil fuel installations,causes them to open on a flow modulating basis whenever hot reheat linesteam pressure exceeds a first set point which follows continuously andautomatically the actual operating pressure, and causes fast full valveopening when reheater pressure exceeds a second set point which alsocontinuously and automatically follows the actual operating pressure,while water flow control unit 88 is arranged to respond to steamtemperature downstream of the desuperheater as evaluated with a steamtemperature sensor, which is not shown in the drawing.

In the Siemens system desuperheating water is admitted into the valvebody itself (71).

In both Sulzer and Siemens systems, as probably in other competingversions, by-pass valves are equipped with hydraulic operators which arecapable of ensuring that rise of steam pressure within the hot reheatline is sufficiently restricted to prevent opening of reheat pressuresafety valves when the steam acceptance of the low pressure turbine isreduced in a fraction of a second by an amount that does not exceed theflow capacity of the valves.

Apparently, also, the valves of by-pass systems used extensively inGermany, and also rather generally in continental Europe, are notsubject to steam leakage in an amount that represents a serious problem,since, per reference 66, practically all thermal power plants incontinental Europe are equipped with HP and LP by-pass systems (cf ref.66 section 5.5 p. 107).

However, in any case, to the extent that leakage should turn out to be aproblem it can, if it is serious enough, be corrected by valvemaintainance, which can be carried out without scheduling a shutdown ifshut off valves are provided on both sides of the valve.

Referring to FIG. 6 when turbine control system 39 receives a fastvalving signal that has been generated in fast valving signal generator33.Iadd., it can be provided for that .Iaddend.there will come intoeffect preprogrammed processes of

1. intercept valve closure which are fast enough and sufficient inextent to have a favorable effect on generator rotor first swingstability, and preferably take the form of full closure effected in 1/4second or less,

2. subsequent reopening of some or all intercept valves to a partiallyopen position with reopening preferably initiated somewhat in advance ofthe first forward swing of the generator rotor, and carried tocompletion within a fraction of a second following the peak of thatswing,

coupled with

3. preprogrammed servo valve implemented retention of partially openedintercept valves in, or substantially in, the preprogrammed positionthat they attained in their rapidly executed reopening process,

with which techniques the intercept valves assume and retain a partiallyopened position at the end of a preprogrammed repositioning cycle, untilsuch time as an election is made to further reopen them.

With use of a by-pass system that responds to reheat pressure, such asshown in FIG. 6, when the by-pass system will pass, at rated full loadhot reheat line pressure, x percent of intermediate pressure steamacceptance at rated full load, it is possible to fully close interceptvalves at top speed, in response to a fast valving signal, and partiallyreopen at or about the instant of generator rotor first forward swing,to a point at which the valves will pass 100-x percent of rated fullload steam flow, and yet avoid lifting of reheat pressure safety valves.

Thus, in detail, assuming by way of example that x=50, and that, as acase in point, the rated full load combined driving power of theintermediate and low pressure turbines is 70 percent of rated load, andtaking it as a sufficient approximation that turbine driving power isproportional to steam flow, it would be possible to reduce low pressureturbine driving power, on a sustained basis, by 35 percent of rated fullload value, by restricting the extent of intercept valve lifting to 50percent on a flow basis, and do so without initiating any process ofclosure of control valves or need to reduce rate of steam generation, orto be concerned about the temperature of the reheater.

Also since pressure ahead of the high pressure turbine will not change,and reheater pressure will change only slightly, total turbine drivingpower will remain substantially fixed at, 100-35=65 percent of ratedfull load value until such time as a decision is made to increasedriving power, which can be accomplished by merely further opening theturbine's intercept valves.

There would also be opportunity to close control valves (17), inresponse to the fast valving signal, at top speed, and fully reopen on apreprogrammed basis, at, or about, or prior to the time of peaking ofthe first forward swing of the generator rotor, and, if high pressuresafety valves are not set too low, their lifting would be avoided, thistending especially to be true if there were available one or more poweroperated relief valves 42.

At the same time there would be the option of reducing rate of steamgeneration subsequent to completion of processes of preprogrammed fastvalving, whereby to allow partial closure of control valves and morecomplete opening of intercept valves, as a way to minimize turbinethermal fatigue effects, if it turned out that such effects were greatenough to warrant.

Evidently, by choosing x=60 turbine driving power could be reduced to100-0.7×60 or 58 percent while with x=40 the figure would be 72 percent,and with x=30, 79 percent.

Also, to the extent that there was capacity to discharge steam toatmosphere, via valves 42, or through other valves that would dischargeto the condenser, or, as in continental European practice, to the coldreheat line (cf refs. 35 and 65), sustained partial control valveclosure could be made use of as a way to minimize needed capacity of thelow pressure by-pass sytem, and to minimize thermal fatigue effectswithin the intermediate pressure turbine.

Also, if by-passing of high pressure steam was accomplished with acontinental Europen type desuperheating by-pass system that divertedsteam from a point ahead of the turbine to the cold reheat line, therewould be no requirement to adjust rate of steam generation since thereheater would be prevented from overheating.

Further, although the control scheme just set forth, was illustrated inthe context of FIG. 1, it will be readily appreciated that it is equallycapable of use as a control system modification that could apply to thesteam electric installations of FIGS. 2 and 3, so long as the interceptvalves are servo controlled and therefore capable of being held fixed ina partially open position.

In relation to all of the foregoing, it is to be understood that, withthe benefit of oil accumulators, fast partial reopening of interceptvalves is capable of accomplishment, either with use of high capacityservo valves, or where size of servos is inadequate, with the use eitherof the metering cylinder system illustrated in FIGS. 4 and 5, or withthe aid of cam operated decelerating type valves.

Where, as matters stand, in PWR nuclear steam electric installations, ithas become the custom to provide, as a minimum, 45 percent high pressuresteam by-pass capability, it would not be necessary to employ a lowpressure steam by-pass system, since, as there is no problem ofoverheating of reheaters, nor, since fast valving would be seldominvoked, any significant problem of thermal fatigue, and since, in a PWRinstallation, momentary closure of control valves does not induce safetyvalve lifting, there is also no problem in effecting a combination ofpreprogrammed fast intercept valve closure followed by partialreopening, coupled with simultaneously executed preprogrammed

a. fast full control valve closure followed by fast partial reopening,or

b. merely fast partial control valve closure, which closure process canbe so carried out as to limit the extent of rise in MSR pressure, andavoid operation of safety valves.

When it comes to either PWR or BWR type nuclear installations, there isan advantage in avoiding reduction of MSR pressure with a view toeliminating hazard of instability of MSR drain systems.

Thus referring to FIG. 3, to achieve this result in a PWR installation,care would be taken that the process of control valve repositioningwould be so programmed that up to the point that the intercept valves.[.(22).]. .Iadd.22 .Iaddend.had reached their preprogrammed finalsustained partially open position, the supply of steam to the moistureseparator reheater .[.(20).]. .Iadd.20 .Iaddend.would exceed the amountleaving it, with the net effect that MSR pressure would at first riserather than fall.

However, at the same time, control valves .[.(17).]. .Iadd.17.Iaddend.would preferably be fully closed and partly reopened, atsomewhat less speed than would apply to the intercept valves .[.(22).]..Iadd.22.Iaddend., as a way to minimize the extent of MSR pressure risethat takes place up to the point of stabilization of the turbine'sintercept valves in their preprogrammed partially reopened position.

Also the option would be available of providing the turbine's controlsystem with an MSR pressure input which input would be arranged so as tocause the control system to hold MSR pressure constant.

As previously noted, in the case of BWR type installations, it isdesirable to dispense with preprogrammed control valve closure, in viewof the fact that reduction of high pressure turbine steam acceptanceoperates to increase reactor pressure, which, if occurring rapidly,other than in small amount, will cause reactor scram.

Further, it is desirable to minimize MSR pressure changes, not only toavoid MSR drain system transients, but also to avoid inducing changes inhigh pressure turbine steam acceptance.

Therefore the desirability of making use of a servo controlled lowpressure by-pass system that responds to rise in MSR pressure isindicated.

In .[.BNR.]. .Iadd.BWR .Iaddend.installations holding control valveposition constant would have the effect that reduction of high pressureturbine steam acceptance would be confined to that induced by increasein pressure ahead of the low pressure turbine, which would be held smallby the low pressure by-pass system, while, also such decrease in highpressure turbine steam acceptance as would be experienced, would bedealt with through the working of the reactor's steam pressure riseresponsive feed back type control system, which regulates the positionof by-pass valves that discharge high pressure steam to the condenser,and which automatically comes into play when high pressure turbine steamacceptance decreases, and operates successfully to prevent reactor scramif rate and extent of pressure change is not excessive.

While the low pressure by-pass control system shown in FIG. 6 offers theadvantage that it holds pressure ahead of intercept valves constant,independent of the extent of preprogrammed, or subsequent further, valvereopening, the point applies that, in the case of fossil fuel and HTGRnuclear installations, it would also be possible to dispense with themodulating aspect of the system, and arrange merely to fully open valveor valves 82 when fast valving is .[.involved,.]. .Iadd.invoked,.Iaddend.and to provide so that the capacity of the valves was suitablycoordinated with the preprogrammed extent of intercept valve partialreopening, or vice versa, in such manner as to hold hot reheat linepressure constant or cause it to somewhat decrease.

If there were several valves 82 there could be election to open avariable number of them, and in any given case, provide for coordinationof extent of preprogrammed intercept valve reopening.

It would be also possible to utilize as by-pass valves 82 air liftedspring loaded valves that would preferably be equipped with shutoffvalves on either side to avoid need for a unit shut down whenmaintenance was required.

Shut-off valves can advantageously be of the gate type while the valvethat stands between the by-pass valve and the hot reheat line wouldpreferably be provided with a split gate having a connection through thevalve body to the space between the two halves of the split gate, whichfeature offers the advantage that it makes possible entire prevention ofleakage of steam through the valve during maintenance periods, by, insuch periods, employing the expedient of pressurizing the space betweenthe gates with high pressure nitrogen.

In another approach to controlling reheat line pressure it would bepossible to arrange to open and close some of reheat pressure valves 82in response to steam pressure, whereby to employ the valves in questionto effect on-off type pressure control.

Where reference has been made to increase in hot reheat line and MSRpressure when fast valving is invoked, and a low-pressure by-pass systemis employed that is arranged to modulate on a feed back basis inresponse to pressure ahead of the system, or if resort is had to theon-off type control, the point applies that control action is notrequired until fast valving is initiated, which means that the setpoints of the controls can be made such as to minimize pressure risewhen the by-pass system comes into effect in response to a fast valvingsignal.

Where, as in FIG. 3, PWR and BWR nuclear installations incorporate threelow pressure turbines it is possible, as an alternate to fast closureand partial reopening of all intercept valves (22), to first close allvalves, but fast reopen the valves of two low pressure turbines whileretaining the valves of the third unit in closed position.

This technique would offer the advantage of allowing .[.extension of.].application of the practice of sustained reduction of driving power oflow pressure turbines of Westinghouse manufacture used in PWR and BWRnuclear installations, in which servo valve control of intercept valvesare not provided.

Also, when, as in the case of GE turbines, there has been, or would be,objection to employing fast closure of intercept valves of PWR and BWRnuclear installation turbines, where this would be followed by fastreopening against full MSR pressure, it would be feasible, instead toprovide for fast closing, and subsequently to hold closed the interceptvalves of one unit while, where needed, bringing about improvement ofgenerator rotor first swing, or first and second swing stability withthe benefit of a braking resistor.

This would leave the problem of how to proceed to reopen the valves ofthe unit that was valved down. However, this could be handled by partlyunloading the turbine, as during nighttime system light load operation,at which point there would be no harm in initiating the reopeningprocess.

Also the above procedures could be extended to fast valving down threelow pressure units and restoring power to only one, or to fast valvingdown and holding two units in unloaded condition, and the conceptsinvolved could also be utilized when there were two instead of three lowpressure turbines, or if there should ever be as many as four or more.

There are thus several ways and techniques of implementing the presentinvention, preferred forms of which vary in some degree in dependence onthe type of turbine steam supply source as well as on the nature of theturbine and its control system.

While the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

In addition it could be in order to note that the point applies thatconcepts of the present invention can be usefully applied as ways toimplement processes of fast valving for the purpose of reducing theextent of turbine overspeeding in the event of partial loss of load,such as can occur in the event of a system breakup that causes systemelectrical islands to form which contain turbine driven generators, andin which the islanding process has resulted in a loss of islanded areaload, either at once, or as a result of the influence of load sheddinginternal to the area (72).

CLAIM TERMINOLOGY

The basic approach to improvement of power system stability thatunderlies what is set forth in the present application is to provide andutilize means of responding to suddenly occurring events that jeopardizepower system stability by sufficiently rapidly reducing the drivingpower of at least one power system generator prime mover.

To accomplish this result delay in initiating valve closure and time toclose, once closure is begun, are preferably made such that valves closefully in 1/4 second or less.

Going back into history, in the Staege patent (1) wording of claim 2 is"2. In a power-transmission system, a power circuit, a generatorconnected to said power circuit, a prime mover for driving saidgenerator, and means for increasing the stability of said transmissionsystem comprising means operative upon abnormal power-circuit currentfor reducing the flow of motive fluid to said prime mover."

Subsequently in the Criscom and Wagner patent (3) in claim 5 thestatement is

"5. In a transmission system, a synchronous generator, a transmissionline connected thereto, said line having a fault-responsivesectionalizing means, a prime-mover for supplying mechanical power tosaid generator, and electric fault-responsive means for temporarilyaltering the available generator-turning power delivered to saidgenerator within a time which is small in comparison to the half-periodof oscillation of the system, the direction of alteration being such asto reduce said oscillation."

In the case of the Staege patent the disclosure calls for restoringdriving power to its predisturbance value after a "predetermined time."

In the case of the Griscom and Wagner patent both the disclosure and allclaims that relate to driving power reduction refer to "temporarily"altering or reducing it or words to that general effect.

In 1929 and 30, at the writer's suggestion, the GE Co. carried out testson a 50,000 kw reheat type turbine generator which demonstrated thefeasibility of employing very rapid momentary reduction of turbinedriving power as a way to improve power system stability (2).

When the writer first filed the patent application that led in duecourse to U.S. Pat. No. 3,051,842, he at first gave consideration onlyto new ways to make use of application of an artificial or braking loadand fast momentary reduction of turbine driving power (cf claims 1through 15), but before the patent issued he modified it to also includeessentially what is covered in claim 2 of Griscomb and Wagner with theadded provision that

". . . the fault is caused to effect a modification of generator primemover driving power characteristics whereby it results that followingclearance of the fault and return to steady power flow conditions theamount of power transmitted over the transmission system from thegenerating segment to the receiving segment is reduced relative toconditions obtaining just prior to the fault." (cf column 3 lines 1through 8).

What the writer had added was the very important concept of effecting adriving power reduction that was not merely rapid enough to favorablyeffect power system stability during the first forward swing of thegenerator rotor following a line fault, which is to say within the firsthalf-period of oscillation of the system, as in Griscom and Wagner, butthat also operated to hold driving power in the post fault period belowits prefault value.

What Staege, and Griscom and Wagner, proposed was basically new, andwhat the writer added was a basically new improvement over what theydisclosed and claimed.

However when the writer was in process of writing the claims of U.S.Pat. No. 3,051,842 he had not seen either the Staege, or Griscom andWagner patents, and did not find himself equal to the task of writingthe kind of brief strong claims that those patents incorporated, andabove all he was unable to argue successfully with the then examinerthat it would not, in the light of the prior art of turbine control, beobvious to anyone skilled in the combined arts of power transmission andturbine control to provide to restrict the extent of preprogrammedreturn of driving power to prefault value during the post fault period.

However as experience well demonstrated, it was not, in fact, obvious,nor, as brought out in the petition to allow a reissue of the writer'ssecond patent, was it obvious that, as proposed in that patent, thereare special advantages in combined employment of fast .[.value.]..Iadd.valve .Iaddend.action of the sustained reduction of driving powertype, and mometary application of a braking load.

What has been at stake is that the writer proposed two entirely newtechniques of improving power system stability, which can be brieflycharacterized as

a. sustained reduction of driving power type fast valving, or moresimply "sustained type fast valving" and

b. the combination of sustained type fast valving and braking,

which, from the start offered an important potential to allow eitherimproving reliability of bulk power transmission, or minimizing need toconstruct power transmission lines.

However it turned out to be very difficult to evoke interest in theseconcepts because of what the writer has termed "the power technologyeducation gap," which relates to the fact that, almost without fail,mechanical engineers do not understand what determines power systemstability, and electrical engineers usually know relatively little aboutsteam turbines and steam generators.

Knowledge in these areas has tended to be closely compartmented, thisbeing somewhat more true in the U.S. than on the continent of Europe,where turbine producers have tended to produce boilers as well.

As the writer began to work toward the implementation of his concept ofuse of very rapidly effected sustained type reduction of turbine drivingpower as a means of improvement of system stability, he encountered thesituation that, whereas power transmission system planners could beeasily convinced of the merit of what he proposed as a way to improvestability and minimize need to build lines, turbine and boiler peopleraised objections which, in time sequence, were to the effects that whatwas proposed would

A. cause lifting of and damage to safety valves,

B. cause objectionable thermal fatigue effects,

C. require changes to turbine control systems,

D. cause objectionable steam generator transients,

E. cause

1. greater control complexity,

2. additional duty on intercept valves,

3. more severe duty on intercept valves,

4. burden of drain system instability,

5. certain blowing of MSR pressure relief valves,

item E above representing the recently stated position of the GE Co'sturbine people as regards fast valving generally, whether of themomentary or sustained reduction of driving power type.

Because of these objections it became necessary to find remedies.

Also, as time went on, refinements were added, such as provision to atfirst employ momentary fast valving, but, in certain situations, withina fraction of a second, convert to sustained type (46) or vice versa(68), and also to alter the extent of preprogrammed sustained drivingpower reduction with a fraction of a second of initiation of fastvalving.

The present application deals with a particular set of ways of avoidingadverse effects on safety valves, turbine valves, and steam generatorswhen fast valving of the sustained reduction of driving power type ismade use of as a way to preserve system stability when threatened byevents such as, but not limited to, line faults, when the event couldoperate to cause instability.

Therefore it depends on use of the basically new concept of respondingto suddenly occurring events that could adversely affect power systemstability by doing things that initiate a sustained type reduction ofturbine driving power that is adapted to take place fast enough to servethe purpose of helping to prevent development of system instability.

When it comes to the hardware, naturally each turbine producer prefersto utilize the hardware that he already has developed and is supplying.

In the U.S., GE and Westinghouse use valve actuator oil dump valves toeffect very rapid valve closure, and these dump valves have to be closedbefore reopening can begin.

GE's dump valves reclose much more slowly than those of Westinghouse,but this is merely objectionable and not necessarily fatal to successespecially when braking is utilized.

Continental European turbines, or at least those of the Brown BoveriCo., use large servo valves as a way to cause fast valve closure.

Westinghouse large nuclear turbines are especially in need of provisionto rapidly reopen their butterfly type valves and, at the instance ofthe TVA, Westinghouse is currently in process of providing for fastopening.

To attain desirable speeds of opening, oil pumps such as are normallyemployed to supply oil for valve reopening purposes, have to besupplemented by oil accumulators.

Also, in order to add the useful feature of fast partial reopening ofintercept valves, if large enough servo valves are not available, theymust be provided, or otherwise cam operated decelerating valves ormetering cylinders as described in this application can be used.

Also where intercept valves require to be held in a partly open positionon a sustained basis, ability to so operate without valve damage isrequired.

The details of how these things can be done, and are being done in thecase of TVA steam-electric installations, are unimportant because thereare many ways to proceed and each turbine control system designer andeach valve designer is free to use whatever approach in his opinionsuits best in his case.

In the course of conversations with the key turbine control and valvedesigners of GE, Westinghouse, and Brown Boveri, except when it came toprovision for fast opening of large valves of nuclear turbines againstfull MSR pressure, there has never been any question as to thefeasibility of providing features that the writer has called for if only

a. there is a genuine economic advantage in their use,

b. adverse effects contingent on their use can be avoided.

With this in mind, and to render the claims easy to read, it hasappeared advantageous to employ a claim wording that does not go intodetail when it comes to what in the prior art is available and would bemade use of.

Griscom and Wagner patent claim number 5 used the terms

". . . means for temporarily altering the available generator-turningpower delivered to said generator within a time which is small incomparison to the half-period of oscillation of the system . . . "

In the context of the present application this wording could beparaphrased to read as below

--means for effecting a sustained reduction of the driving power of theturbine within a time period which is small in comparison with the halfperiod of oscillation of the system--

However this phraseology would be too narrow

a. first because ordinarily it would be desirable for driving power tosustain at approximately the value that would be arrived at at the endof the half period of oscillation and

b. second because as brought out in references 23, 46, and 68 there areexceptions to the rule.

Thus what needs to be done varies from situation to situation, whilealso as brought out in references 23, 31, and 32, it can be advantageousto boost rather than reduce driving power.

In general there are several situations in which it is useful toreposition turbine valves as rapidly as possible, and it follows thatthere can be an advantage in providing a common way to characterize thistype of valve action.

To have a simple terminology the words fast turbine valving have seemedbest to fit the bill (64).

Historically the first, and still the most important, purpose of fastvalving has been to minimize development of overspeed, on full orpartial loss of load, by rapidly closing valves.

Also there has always been, and remains, a need to prevent too great adrop in speed when load suddenly increases, by providing to rapidly openvalves.

However, as a new development, fast turbine valving can also be used asa way to avoid development of system instability as a consequence ofstability endangering events, and when so used can be employed in twoways, as below,

a. to reduce turbine driving power by closing valves, in the case ofdevelopment of stability endangering events of a type adapted to cause agenerator to experience a sudden at least momentary reduction of load,

b. to increase turbine driving power by opening valves in the case ofevents of a type adapted to cause a generator to experience a suddenincrease in load

while those skilled in the art of turbine control know how this can bedone, by providing within the turbine's control system for apredetermined response to a fast valving signal input which responsewill comprise a preprogrammed process of valve repositioning.

Now, in the above, it is to be understood that a turbine's controlsystem can incorporate more than one preprogrammed process of fastvalving in response to a fast valving signal, as is well brought out inreference 54, which comprises GE U.S. Pat. No. 3,601,617, and in whichprovision is made to effect fast valving, both

a. for overspeed protection, and

b. for improvement in system stability,

with valve behavior dependent on which of two types of fast valvingsignals has been generated, program (a) being made responsive to a fastvalving signal generated as a result of a sudden full or partial loss ofstation load not involving a line fault, and program (b) being maderesponsive to the occurrence of a fault.

Also, as brought out in the writer's various patents and publicationsthat deal with fast valving for stability improvement, the nature of thepreprogrammmed valving cycle can be automatically varied in dependenceon such factors as prefault transmitted load (21), the occurrence ornon-occurrence of a refault after reclosure of faulted line circuitbreakers (22, 23), the trip-off of generators, and the opening ofintersystem ties (31, 32), the development of delay in fault clearance(46, 68), and the type and location of line faults (44, 64), and that inthese connections it can be desirable to provide, as described inreference 64 and 68, so that when a fast valving signal is generated inresponse to a system stability endangering event the signal is madeavailable, selectively, as an input to one of a group of two or moreturbine control system fast valving signal input channels, each ofwhich, when a signal is received, activates a different portion of theturbine's control system, and brings into effect a different type ofpreprogrammed fast valving cycle.

Whereas not made explicit in the claims it is to be understood thatnormally, as shown in the figures, the power delivered by alternatingcurrent generators is stepped up in voltage by generator transformers,and delivered via circuit breakers, supplied to transmission systemswhich serve to interconnect the generator with other generators, whilealso the point will apply that large steam turbines of compound typeinvariably receive their steam from steam generators.

Where, as in some cases, direct current lines or ac-dc-ac back to backconverters are made use of, advantage in fast valving for stabilityimprovement can fail to apply, but this will not be the case where theturbine generator that would be fast valved is united to othergenerators by a plurality of alternating current transmission circuits.

Where in the above, and in the claims, use is made of the wordpreprogrammed, what is to be understood is that where valverepositioning is involved, on energization of some sort of triggerdevice, spring loaded valves will close in a manner that will beentirely determined by the design of the valves, springs, and valveactuator oil discharge means.

Also, it will be provided in advance that the reopening process willbegin at a preset point in time following the end of the closingprocess.

Further there will be advance provision that will determine the natureof the stroke of the valve versus time, during the opening process.

It will also be understood that a preprogrammed process of signalgeneration and control system fast valving input signal channelselection implies advance determination of what will take place.

The word sustained is used in the claims in the context of apreprogrammed process of sustained type partial intercept valvereopening and is to be understood to mean that the valve ends up in apartially open position, and will remain there unless and untilsomething that was not preprogrammed takes place.

In this section on claim terminology, words and phrases that have beenunderlined represent, in a sense, a special language that has proved tobe useful when dealing with fast turbine valving for stabilityimprovement, an area of expertise for which, until recently, there wasno need of specialized language development.

Based on the foregoing what I claim is:
 1. In a power system whichincludes a plurality of prime mover driven generators, which areinterconnected by a plurality of alternating current transmissioncircuits, and which include at least one generator that is driven by acompound type steam turbine incorporating control valves ahead of thehigh pressure turbine, and intercept valves ahead of the turbine orturbines that are driven by steam that is discharged by the highpressure turbine, the method of fast valving of the said compoundturbine, as a way to avoid development of system instability, whichcomprises the steps of1. providing within the turbine's control systemfor a predetermined response to a fast valving signal input whichresponse will bring into effect preprogrammed processes of,a. at leastpartial closure of the turbine's intercept valves, .Iadd.effected fastenough to have a favorable effect on generator rotor first swingstability, .Iaddend. b. sustained type partial intercept valvereopening.
 2. providing in a preprogrammed manner so that,a. a fastvalving signal is generated on the occurrence of .Iadd.certain types of.Iaddend.system stability endangering events .[.of a type.]. that causethe said generator to experience a sudden at least momentary reductionof load, b. the said fast valving signal is made available as an inputto that portion of the turbine's control system that is adapted to bringinto effect the said predetermined response.
 2. The process of claim 1in which intercept valve reopening is initiated within 0.1 second ofcompletion of the closing process.
 3. The process of claim 1 whichpartial intercept valve reopening carried out in step (1-b) is effectedwith oil supplied from accumulators, and is terminated by operation ofvalves of cam operated type, with cam position determined by valvestroke.
 4. The process of claim 1 in which fast partial intercept valvereopening, carried out in step (1-b)is effected by the transfer to thecylinder of each of the valve actuators, of a predetermined quantity ofoil contained within a second cylinder.
 5. The process of claim 1, butsupplemented by provision of preprogrammed fast valving signal initiatedcontrol valve repositioning, accomplished rapidly enough, and sufficientin extent, to prevent an increase of steam pressure ahead of theturbine's intercept valves that if not prevented, would suffice to causedischarge of steam through safety valves located ahead of said interceptvalves.
 6. The process of claim 1 but supplemented by providing so thatreceipt by the turbine's control system of a fast valving signal inputcauses discharge of steam to the condenser through .Iadd.one or more.Iaddend.steam flow control means which are arranged to receive steamfrom a point .Iadd.down stream of the high pressure turbine, but.Iaddend.ahead of the turbine's intercept valves.
 7. In a power systemwhich includes a plurality of prime mover driven generators, which areinterconnected by a plurality of alternating current transmissioncircuits, and which include at least one generator that is driven by anuclear type compound steamm turbine incorporating control valves aheadof the high pressure turbine, and intercept valves ahead of the turbineor turbines that are driven by steam that is discharged by the highpressure turbine, the method of fast valving of the said compoundturbine, as a way to avoid development of system instability, whichcomprises the steps of1. providing within the turbine's control systemfor a predetermined response to a fast valving signal input whichresponse will bring into effect preprogrammed process of,a. at leastpartial closure of the turbine's intercept valves, .[.,.]..Iadd.effected fast enough to have a favorable effect on generator rotorfirst swing stability, .Iaddend. b. sustained type partial interceptvalve reopening, c. control valve repositioning of the at least partialclosure type followed by partial reopening so programmed that, towhatever extent practicable, the time variation of flow of the steam inthe steam and water mixture that enters the separator conforms to thetime variation of steam flow out of the separator, whereby to minimizechange in pressure of steam within the separator,
 2. providing in apreprogrammed manner so that,a. a fast valving signal is generated onthe occurrence .Iadd.of certain types .Iaddend.of system stabilityendangering events .[.of a type.]. that cause the said generator toexperience a sudden at .[.lease.]. .Iadd.least .Iaddend.momentaryreduction of load, b. the said fast valving signal is made available asan input to that portion of the turbine's control system that is adaptedto bring into effect the said predetermined response.
 8. The process ofclaim 7, but supplemented by provision so that receipt by the turbinecontrol system of a fast valving signal input initiates a preprogrammedprocess of reduction of rate of steam production within the steamgenerator.
 9. In a power system which includes a plurality of primemover driven generators, which are interconnected by a plurality ofalternating current transmission circuits, and which include at leastone generator that is driven by a compound type steam turbineincorporating control valves ahead of the high pressure turbine, and inwhich steam discharged from a high pressure turbine passes into two ormore low pressure turbines, each of which is equipped with a complementof intercept valves, which, when closed, prevent access of steam to theturbine to which they connect, the method of fast valving of saidcompound turbine, as a way to avoid development of system instability,which comprises the steps of1. providing within the turbine's controlsystem for a predetermined response to a fast valving signal input whichresponse will bring into effect preprogrammed processes of,a. at leastpartial closure of the intercept valves of all of the installation's lowpressure turbine, .Iadd.effected fast enough to have a favorable effecton generator rotor first swing stability, .Iaddend. b. thereafter,holding in closed position the intercept valves of at least one but notall of the said low pressure turbines, while fully opening the interceptvalves of the balance of the installation's low pressure turbines 2.providing in a preprogrammed manner so that,a. a fast valving signal isgenerated on the occurrence .Iadd.of certain types .Iaddend.of systemstability endangering events .[.of a type.]. that cause the saidgenerator to experience a sudden at least momentary reduction of load,b. the said fast valving signal is made available as an input to thatportion of the turbine's control system that is adapted to bring intoeffect the said predetermined response.
 10. The process of claim 9, butsupplemented by provision of preprogrammed fast valving signal initiatedcontrol valve repositioning, accomplished rapidly enough, and sufficientin extent, to prevent an increase of steam pressure ahead of theturbine's intercept valves that, if not prevented, would suffice tocause discharge of steam through safety valves located ahead of saidintercept valves.
 11. The process of claim 9 but supplemented byproviding so that receipt by the turbine's control system of a fastvalving signal input causes discharge of steam to the condenser throughsteam flow control means which are arranged to receive steam from apoint .Iadd.down stream of the high pressure turbine, but .Iaddend.aheadof the turbine's intercept valves.
 12. In a power system which includesa plurality of prime mover driven generators, which are interconnectedby a plurality of alternating current transmission circuits, and whichinclude at least one generator that is driven by a compound type steamturbine incorporating control valves ahead of the high pressure turbine,and in which steam discharged from a high pressure turbine passes intotwo or more low pressure turbines, each of which is equipped with acomplement of intercept valves, which, when closed, prevent access ofsteam to the turbine to which they connect, the method of fast valvingof the said compound turbine, as a way to avoid development of systeminstability, which comprises the steps of1. providing within theinstallation's turbine control system for a predetermined response to afast valving signal input which response will bring into effect apreprogrammed process of full closure of the intercept valves of atleast one, but not all, of the installation's low pressure turbines,while retaining the intercept valves of the balance of theinstallation's turbines in full open position,
 2. providing in apreprogrammed manner so that,a. fast valving signal is generated on theoccurrence .Iadd.of certain types .Iaddend.of system stabilityendangering events .[.of a type.]. that cause the said generator toexperience a sudden at least momentary reduction of load, b. the saidfast valving signal is made available as an input to that portion of theturbine's control system that is adapted to bring into effect the saidpredetermined response.
 13. The process of claim 12, but supplemented byprovision of preprogrammed fast valving signal initiated control valverepositioning, accomplished rapidly enough, and sufficient in extent, toprevent an increase of steam pressure ahead of the turbine's interceptvalves that, if not prevented, would suffice to cause discharge of steamthrough safety valves located ahead of said intercept valves.
 14. Theprocess of claim 12, but supplemented by providing so that receipt bythe turbine's control system of a fast valving signal input causesdischarge of steam to the condenser through steam flow control meanswhich are arranged to receive steam from a point .Iadd.down stream ofthe high pressure turbine, but .Iaddend.ahead of the turbine's interceptvalves.